Remove the optional salsa dependency from the AST crate

[MichaReiser]

Summary

I added the Lookup implementation to Name to allow Class and FunctionType lookups with a &str.
What I didn't realize is that all lookups have a &Name reference. We never have a case where we just have a &str.

That's why I'm removing the Lookup implementation because I prefer if ruff-python-ast doesn't depend on salsa, even if optionally.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.