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_.)
