Add support for development dependencies (#4036)

## Summary

Externally, development dependencies are currently structured as a flat
list of PEP 580-compatible requirements:

```toml
[tool.uv]
dev-dependencies = ["werkzeug"]
```

When locking, we lock all development dependencies; when syncing, users
can provide `--dev`.

Internally, though, we model them as dependency groups, similar to
Poetry, PDM, and [PEP 735](https://peps.python.org/pep-0735). This
enables us to change out the user-facing frontend without changing the
internal implementation, once we've decided how these should be exposed
to users.

A few important decisions encoded in the implementation (which we can
change later):

1. Groups are enabled globally, for all dependencies. This differs from
extras, which are enabled on a per-requirement basis. Note, however,
that we'll only discover groups for uv-enabled packages anyway.
2. Installing a group requires installing the base package. We rely on
this in PubGrub to ensure that we resolve to the same version (even
though we only expect groups to come from workspace dependencies anyway,
which are unique). But anyway, that's encoded in the resolver right now,
just as it is for extras.

crates/uv-distribution/src/lib.rs

@@ -2,7 +2,7 @@ pub use distribution_database::{DistributionDatabase, HttpArchivePointer, LocalA
 pub use download::LocalWheel;
 pub use error::Error;
 pub use index::{BuiltWheelIndex, RegistryWheelIndex};
-pub use metadata::{ArchiveMetadata, Metadata, RequiresDist};
+pub use metadata::{ArchiveMetadata, Metadata, RequiresDist, DEV_DEPENDENCIES};
 pub use reporter::Reporter;
 pub use workspace::{ProjectWorkspace, Workspace, WorkspaceError, WorkspaceMember};