Steps taken:
- add `dependency-groups==1.3.0` to vendor.txt
- add dependency-groups to vendor __init__.py
- run vendoring sync
- examine results to confirm apparent correctness (rewritten tomli
  imports)

`--group` is supported on `download` and `install` commands.
The option is parsed into the more verbose and explicit
`dependency_groups` name on the parsed args.

Both of these commands invoke the same processor for resolving
dependency groups, which loads `pyproject.toml` and resolves the list
of provided groups against the `[dependency-groups]` table.

A small alteration is made to `pip wheel` to initialize
`dependency_groups = []`, as this allows for some lower-level
consistency in the handling of the commands.

A new unit test module is added for parsing dependency groups and used
to verify all of the pip-defined behaviors for handling
dependency-groups.

In one path, the underlying exception message from `dependency-groups`
is exposed to users, where it should offer some explanation of why
parsing failed, and this is therefore tested.

Some related changes are applied to the dependency groups usage sites
in the src tree. The signature of the dependency group requirement
parse function is simplified, and its usage is therefore updated.
A bugfix is applied to add a missing `f` on an intended f-string.

This initial suite of tests is modeled fairly closely on existing
tests for requirements files.

Tests cover the following cases:
- installing an empty dependency group (and nothing else)
- installing from a simple / straightforward group
- installing from multiple groups in a single command
- normalizing names from the CLI and pyproject.toml to match
- applying a constraints file to a dependency-group install

Per review, support on `pip wheel` is desirable. This is net-net
simpler, since we don't need any trickery to "dodge" the fact that it
is a `RequirementCommand` but wasn't supporting `--group`.

The desire to *not* support `--group` here was based on a mistaken
idea about what `pip wheel` does.