By resolving `.` and the LHS of the from import during semantic indexing, we can check if the LHS is a submodule of `.`, and handle `from whatever.thispackage.x.y import z` exactly like we do `from .x.y import z`. Fixes https://github.com/astral-sh/ty/issues/1484
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.