Previously, we were always asking Cargo to rebuild `uv-cli` if
`.git/HEAD` had changed. But in a worktree, `.git` is a file, not a
directory. And the file contains the path to git's internal worktree
state, which also has its own `HEAD` file. So in the case of a worktree,
we read the file and tell Cargo to watch the worktree-specific `HEAD`
file instead of `.git/head`.
The main thing this fixes is that, previously, in a worktree, `cargo
build` would *always* re-compile `uv` even if nothing changed.
This doesn't impact or fix anything in "typical" clones of uv though.
Only in worktrees.
Closes#6196, Closes#6197
## Summary
This PR moves all the CLI code into its own crate, separate from the
`uv` crate. The `uv` crate is iterated on frequently, and the CLI code
comprises a significant portion of it but rarely changes. Removing the
CLI code reduces the `uv` crate size from 1.4MiB to 1.0MiB.
