Add an in-memory cache for Git references

[charliermarsh]

Summary

Ensures that, even if we try to resolve the same Git reference twice within an invocation, it always returns a (cached) consistent result.

Closes #2673.

Test Plan

❯ cargo run pip install git+https://github.com/pallets/flask.git --reinstall --no-cache
   Compiling uv-distribution v0.0.1 (/Users/crmarsh/workspace/uv/crates/uv-distribution)
   Compiling uv-resolver v0.0.1 (/Users/crmarsh/workspace/uv/crates/uv-resolver)
   Compiling uv-installer v0.0.1 (/Users/crmarsh/workspace/uv/crates/uv-installer)
   Compiling uv-dispatch v0.0.1 (/Users/crmarsh/workspace/uv/crates/uv-dispatch)
   Compiling uv-requirements v0.1.0 (/Users/crmarsh/workspace/uv/crates/uv-requirements)
   Compiling uv v0.1.24 (/Users/crmarsh/workspace/uv/crates/uv)
    Finished dev [unoptimized + debuginfo] target(s) in 3.95s
     Running `target/debug/uv pip install 'git+https://github.com/pallets/flask.git' --reinstall --no-cache`
 Updated https://github.com/pallets/flask.git (b90a4f1)
Resolved 7 packages in 280ms
   Built flask @ git+https://github.com/pallets/flask.git@b90a4f1f4a370e92054b9cc9db0efcb864f87ebe                                                                                                                                            Downloaded 7 packages in 212ms
Installed 7 packages in 9ms