Move 'get_project_imports' to Rust extension [18.8x speedup]

[emdoyle]

By far most of the time spent by tach check or tach sync is in get_project_imports, particularly on large projects.

[this is tach sync on the Sentry codebase (~3000 Python files)]

You can also see in that flamegraph that most of the time spent within import scanning is simply visiting all the nodes (even for essentially generic no-ops) and building the ASTs themselves.

This PR moves the implementation of get_project_imports to our Rust extension library, using the ruff Python AST parser and node visitor crates. The functionality is otherwise generally a direct translation of our Python code.

[this is tach sync on the Sentry codebase (~3000 Python files) using Rust for import scanning]