Add extra dependency annotations to lockfile and sync commands (#3913)
## Summary
This PR adds extras to the lockfile, and enables users to selectively
sync extras in `uv sync` and `uv run`. The end result here was fairly
simple, though it required a few refactors to get here. The basic idea
is that `DistributionId` now includes `extra: Option<ExtraName>`, so we
effectively treat extras as separate packages. Generating the lockfile,
and generating the resolution from the lockfile, fall out of this
naturally with no special-casing or additional changes.
The main downside here is that it bloats the lockfile significantly.
Specifically:
- We include _all_ distribution URLs and hashes for _every_ extra
variant.
- We include all dependencies for the extra variant, even though that
are dependencies of the base package.
We could normalize this representation by changing each distribution
have an `optional-dependencies` hash map that keys on extras, but we
actually don't have the information we need to create that right now
(specifically, we can't differentiate between dependencies that
_require_ the extra and dependencies on the base package).
Closes #3700.