[red-knot] Fix python setting in mdtests, and rewrite a site-packages test as an mdtest (#17222)

## Summary

This PR does the following things:
- Fixes the `python` configuration setting for mdtest (added in
https://github.com/astral-sh/ruff/pull/17221) so that it expects a path
pointing to a venv's `sys.prefix` variable rather than the a path
pointing to the venv's `site-packages` subdirectory. This brings the
`python` setting in mdtest in sync with our CLI `--python` flag.
- Tweaks mdtest so that it automatically creates a valid `pyvenv.cfg`
file for you if you don't specify one. This makes it much more ergonomic
to write an mdtest with a custom `python` setting: red-knot will reject
a `python` setting that points to a directory that doesn't have a
`pyvenv.cfg` file in it
- Tweaks mdtest so that it doesn't check a custom `pyvenv.cfg` as Python
source code if you _do_ add a custom `pyvenv.cfg` file for your mock
virtual environment in an mdtest. (You get a lot of diagnostics about
Python syntax errors in the `pyvenv.cfg` file, otherwise!)
- Rewrites the test added in
https://github.com/astral-sh/ruff/pull/17178 as an mdtest, and deletes
the original test that was added in that PR

## Test Plan

I verified that the new mdtest fails if I revert the changes to
`resolver.rs` that were added in
https://github.com/astral-sh/ruff/pull/17178

crates/red_knot_python_semantic/resources/mdtest/import/relative.md

@@ -236,3 +236,36 @@ X: int = 42
 ```py
 from .parser import X  # error: [unresolved-import]
 ```
+
+## Relative imports in `site-packages`
+
+Relative imports in `site-packages` are correctly resolved even when the `site-packages` search path
+is a subdirectory of the first-party search path. Note that mdtest sets the first-party search path
+to `/src/`, which is why the virtual environment in this test is a subdirectory of `/src/`, even
+though this is not how a typical Python project would be structured:
+
+```toml
+[environment]
+python = "/src/.venv"
+python-version = "3.13"
+```
+
+`/src/bar.py`:
+
+```py
+from foo import A
+
+reveal_type(A)  # revealed: Literal[A]
+```
+
+`/src/.venv/<path-to-site-packages>/foo/__init__.py`:
+
+```py
+from .a import A as A
+```
+
+`/src/.venv/<path-to-site-packages>/foo/a.py`:
+
+```py
+class A: ...
+```

crates/red_knot_python_semantic/src/db.rs

@@ -19,7 +19,7 @@ pub(crate) mod tests {
     use std::sync::Arc;
 
     use crate::program::{Program, SearchPathSettings};
-    use crate::{default_lint_registry, ProgramSettings, PythonPath, PythonPlatform};
+    use crate::{default_lint_registry, ProgramSettings, PythonPlatform};
 
     use super::Db;
     use crate::lint::{LintRegistry, RuleSelection};
@@ -139,8 +139,6 @@ pub(crate) mod tests {
         python_version: PythonVersion,
         /// Target Python platform
         python_platform: PythonPlatform,
-        /// Paths to the directory to use for `site-packages`
-        site_packages: Vec<SystemPathBuf>,
         /// Path and content pairs for files that should be present
         files: Vec<(&'a str, &'a str)>,
     }
@@ -150,7 +148,6 @@ pub(crate) mod tests {
             Self {
                 python_version: PythonVersion::default(),
                 python_platform: PythonPlatform::default(),
-                site_packages: vec![],
                 files: vec![],
             }
         }
@@ -169,14 +166,6 @@ pub(crate) mod tests {
             self
         }
 
-        pub(crate) fn with_site_packages_search_path(
-            mut self,
-            path: &(impl AsRef<SystemPath> + ?Sized),
-        ) -> Self {
-            self.site_packages.push(path.as_ref().to_path_buf());
-            self
-        }
-
         pub(crate) fn build(self) -> anyhow::Result<TestDb> {
             let mut db = TestDb::new();
 
@@ -186,15 +175,12 @@ pub(crate) mod tests {
             db.write_files(self.files)
                 .context("Failed to write test files")?;
 
-            let mut search_paths = SearchPathSettings::new(vec![src_root]);
-            search_paths.python_path = PythonPath::KnownSitePackages(self.site_packages);
-
             Program::from_settings(
                 &db,
                 ProgramSettings {
                     python_version: self.python_version,
                     python_platform: self.python_platform,
-                    search_paths,
+                    search_paths: SearchPathSettings::new(vec![src_root]),
                 },
             )
             .context("Failed to configure Program settings")?;

crates/red_knot_python_semantic/src/lib.rs

@@ -10,6 +10,7 @@ pub use module_resolver::{resolve_module, system_module_search_paths, KnownModul
 pub use program::{Program, ProgramSettings, PythonPath, SearchPathSettings};
 pub use python_platform::PythonPlatform;
 pub use semantic_model::{HasType, SemanticModel};
+pub use site_packages::SysPrefixPathOrigin;
 
 pub mod ast_node_ref;
 mod db;

crates/red_knot_python_semantic/src/types/infer.rs

@@ -7392,7 +7392,7 @@ impl StringPartsCollector {
 
 #[cfg(test)]
 mod tests {
-    use crate::db::tests::{setup_db, TestDb, TestDbBuilder};
+    use crate::db::tests::{setup_db, TestDb};
     use crate::semantic_index::definition::Definition;
     use crate::semantic_index::symbol::FileScopeId;
     use crate::semantic_index::{global_scope, semantic_index, symbol_table, use_def_map};
@@ -7400,7 +7400,7 @@ mod tests {
     use crate::types::check_types;
     use ruff_db::diagnostic::Diagnostic;
     use ruff_db::files::{system_path_to_file, File};
-    use ruff_db::system::{DbWithWritableSystem as _, SystemPath};
+    use ruff_db::system::DbWithWritableSystem as _;
     use ruff_db::testing::{assert_function_query_was_not_run, assert_function_query_was_run};
 
     use super::*;
@@ -7556,26 +7556,6 @@ mod tests {
         Ok(())
     }
 
-    #[test]
-    fn relative_import_resolution_in_site_packages_when_site_packages_is_subdirectory_of_first_party_search_path(
-    ) {
-        let project_root = SystemPath::new("/src");
-        let foo_dot_py = project_root.join("foo.py");
-        let site_packages = project_root.join(".venv/lib/python3.13/site-packages");
-
-        let db = TestDbBuilder::new()
-            .with_site_packages_search_path(&site_packages)
-            .with_file(&foo_dot_py, "from bar import A")
-            .with_file(&site_packages.join("bar/__init__.py"), "from .a import *")
-            .with_file(&site_packages.join("bar/a.py"), "class A: ...")
-            .build()
-            .unwrap();
-
-        assert_file_diagnostics(&db, foo_dot_py.as_str(), &[]);
-        let a_symbol = get_symbol(&db, foo_dot_py.as_str(), &[], "A");
-        assert!(a_symbol.expect_type().is_class_literal());
-    }
-
     #[test]
     fn pep695_type_params() {
         let mut db = setup_db();