[ty] Move `fixes.rs` to `ty_python_semantic` (#24561)
## Summary
This is in preparation for https://github.com/astral-sh/ruff/pull/24097
I want to integrate fixes into mdtests, but `ty_test` doesn't depend on
`ty_project`, and adding this new dependency would create more cyclic
dependencies between crates (not targets, just crates), which r-a
sometimes struggles with.
The solution in this PR is to move `fixes` to `ty_python_semantic`. This
is mostly a trivial change, except that it requires adding a
`check_file` method to `db.`
## Test Plan
`cargo test`
