Move source distribution unpacking out of build (#2294)

## Summary

If a user provides a source distribution via a direct path, it can
either be an archive (like a `.tar.gz` or `.zip` file) or a directory.
If the former, we need to extract (e.g., unzip) the contents at some
point. Previously, this extraction was in `uv-build`; this PR lifts it
up to the distribution database.

The first benefit here is that various methods that take the
distribution are now simpler, as they can assume a directory.

The second benefit is that we no longer extract _multiple times_ when
working with a source distribution. (Previously, if we tried to get the
metadata, then fell back and built the wheel, we'd extract the wheel
_twice_.)

crates/uv-dev/src/build.rs

@@ -28,7 +28,7 @@ pub(crate) struct BuildArgs {
     /// Directory to story the built wheel in
     #[clap(short, long)]
     wheels: Option<PathBuf>,
-    /// The source distribution to build, either a directory or a source archive.
+    /// The source distribution to build, as a directory.
     sdist: PathBuf,
     /// The subdirectory to build within the source distribution.
     subdirectory: Option<PathBuf>,