[red-knot] Trust all symbols in stub files (#17588)

## Summary

*Generally* trust undeclared symbols in stubs, not just at the module
level.

Follow-up on the discussion
[here](https://github.com/astral-sh/ruff/pull/17577#discussion_r2055945909).

## Test Plan

New Markdown test.

crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md

@@ -292,3 +292,66 @@ reveal_type(a)  # revealed: Unknown
 # Modifications allowed in this case:
 a = None
 ```
+
+## In stub files
+
+In stub files, we have a minor modification to the rules above: we do not union with `Unknown` for
+undeclared symbols.
+
+### Undeclared and bound
+
+`mod.pyi`:
+
+```pyi
+MyInt = int
+
+class C:
+    MyStr = str
+```
+
+```py
+from mod import MyInt, C
+
+reveal_type(MyInt)  # revealed: Literal[int]
+reveal_type(C.MyStr)  # revealed: Literal[str]
+```
+
+### Undeclared and possibly unbound
+
+`mod.pyi`:
+
+```pyi
+def flag() -> bool:
+    return True
+
+if flag():
+    MyInt = int
+
+    class C:
+        MyStr = str
+```
+
+```py
+# error: [possibly-unbound-import]
+# error: [possibly-unbound-import]
+from mod import MyInt, C
+
+reveal_type(MyInt)  # revealed: Literal[int]
+reveal_type(C.MyStr)  # revealed: Literal[str]
+```
+
+### Undeclared and unbound
+
+`mod.pyi`:
+
+```pyi
+if False:
+    MyInt = int
+```
+
+```py
+# error: [unresolved-import]
+from mod import MyInt
+
+reveal_type(MyInt)  # revealed: Unknown
+```

crates/red_knot_python_semantic/src/semantic_index/symbol.rs

@@ -115,10 +115,6 @@ impl<'db> ScopeId<'db> {
         self.node(db).scope_kind().is_function_like()
     }
 
-    pub(crate) fn is_module_scope(self, db: &'db dyn Db) -> bool {
-        self.node(db).scope_kind().is_module()
-    }
-
     pub(crate) fn is_type_parameter(self, db: &'db dyn Db) -> bool {
         self.node(db).scope_kind().is_type_parameter()
     }
@@ -267,10 +263,6 @@ impl ScopeKind {
         matches!(self, ScopeKind::Class)
     }
 
-    pub(crate) fn is_module(self) -> bool {
-        matches!(self, ScopeKind::Module)
-    }
-
     pub(crate) fn is_type_parameter(self) -> bool {
         matches!(self, ScopeKind::Annotation | ScopeKind::TypeAlias)
     }

crates/red_knot_python_semantic/src/symbol.rs

@@ -593,7 +593,7 @@ fn symbol_by_id<'db>(
                 "__slots__" | "TYPE_CHECKING"
             );
 
-            if scope.is_module_scope(db) && scope.file(db).is_stub(db.upcast()) {
+            if scope.file(db).is_stub(db.upcast()) {
                 // We generally trust module-level undeclared symbols in stubs and do not union
                 // with `Unknown`. If we don't do this, simple aliases like `IOError = OSError` in
                 // stubs would result in `IOError` being a union of `OSError` and `Unknown`, which