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Support for uv lockfiles as an alternative to pex lockfiles (#23302)

uv lockfiles are first-class entities that users check in.
When a pex is needed, we use uv to create a venv, which
it does very efficiently, and then use pex's
`--venv-repository` feature to build a pex from
(a subset of) that venv's requirements.

Note that we still create VenvPexes to run tools, and
in some cases this might be unnecessary, since we
already have the uv venv. But utilizing those venvs 
directly would be an even bigger change (and this one
is already massive), and there are caching tradeoffs
to figure out . So we leave that for the future (and
probably for the new python  backend).

Note also that the dynamics of lockfile generation remain
pants's (you must generate explicitly) and not uv's
(lockfiles are automatically updated for you when
input requirements change). The latter is desirable, but
again this is for the future.

The general thrust of the changes is:

- Add uv as a third lockfile format (after pex and the deprecated
  legacy constraints file "locking")
- Update the lockfile metadata to include the format (and
  also the name of the resolve that created the lockfile).
- Make lockfile metadata reading much more robust 
  and useful, so we can get the format from it reliably.
  Unfortunately this logic is tricky since it has to handle
  pex lockfiles both with and without separate metadata files.
  For uv we simply require separate metadata files and do
  not add any metadata inside the lockfile itself, as we
  foolishly did for pex lockfiles at one point.
- Update the lockfile diffing code to support diffing
  across all combinations of old/new and pex/uv.
- Update the existing `uv` subsystem for more general use.
- Add src/python/pants/backend/python/util_rules/uv.py, 
  which runs `uv`, first generating ephemeral pyproject.toml
  and uv.toml config files. We handle environments correctly, 
  which requires some `realpath` shenanigans.
- Update some iffy ... (continued)

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