d47088c8f8
This causes the diagnostic to highlight the actual unresovable import instead of the entire `from ... import ...` statement. While we're here, we expand the test coverage to cover all of the possible ways that an `import` or a `from ... import` can fail. Some considerations: * The first commit in this PR adds a regression test for the current behavior. * This creates a new `mdtest/diagnostics` directory. Are folks cool with this? I guess the idea is to put tests more devoted to diagnostics than semantics in this directory. (Although I'm guessing there will be some overlap.) Fixes #15866
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/red_knot_test/README.md for documentation of this test format.