Validate lockfile (rather than re-resolve) in uv lock (#6091)

## Summary

Historically, in order to "resolve from a lockfile", we've taken the
lockfile, used it to pre-populate the in-memory metadata index, then run
a resolution. If the resolution didn't match our existing resolution, we
re-resolved from scratch.

This was an appealing approach because (in theory) it didn't require any
dedicated logic beyond pre-populating the index. However, it's proven to
be _really_ hard to get right, because it's a stricter requirement than
we need. We just need the current lockfile to _satisfy_ the requirements
provided by the user. We don't actually need a second resolution to
produce the exact same result. And it's not uncommon that this second
resolution differs, because we seed it with preferences, which
fundamentally changes its course. We've worked hard to minimize those
"instabilities", but they're still present.

The approach here is intended to be much simpler. Instead of resolving
from the lockfile, we just check if the current resolution satisfies the
state of the workspace. Specifically, we check if the lockfile (1)
contains all the relevant members, and (2) matches the metadata for all
dependencies, recursively. (We skip registry dependencies, assuming that
they're immutable.)

This may actually be too conservative, since we can have resolutions
that satisfy the requirements, even if the requirements have changed
slightly. But we want to bias towards correctness for now.

My hope is that this scheme will be more performant, simpler, and more
robust.

Closes https://github.com/astral-sh/uv/issues/6063.

crates/uv-resolver/src/lib.rs

@@ -3,7 +3,7 @@ pub use error::{NoSolutionError, NoSolutionHeader, ResolveError};
 pub use exclude_newer::ExcludeNewer;
 pub use exclusions::Exclusions;
 pub use flat_index::FlatIndex;
-pub use lock::{Lock, LockError, TreeDisplay};
+pub use lock::{Lock, LockError, ResolverManifest, TreeDisplay};
 pub use manifest::Manifest;
 pub use options::{Options, OptionsBuilder};
 pub use preferences::{Preference, PreferenceError, Preferences};