Implement --package for pip tree (#4655)

## Summary

Part of https://github.com/astral-sh/uv/issues/4439.

## Test Plan

The existing tests pass + added a couple of tests to ensure `--package` behaves as expected.

crates/uv-cli/src/lib.rs

@@ -1428,6 +1428,11 @@ pub struct PipTreeArgs {
     /// Prune the given package from the display of the dependency tree.
     #[arg(long)]
     pub prune: Vec<PackageName>,
+
+    /// Display only the specified packages.
+    #[arg(long)]
+    pub package: Vec<PackageName>,
+
     /// Do not de-duplicate repeated dependencies.
     /// Usually, when a package has already displayed its dependencies,
     /// further occurrences will not re-display its dependencies,