(This is the CNB equivalent of heroku/heroku-buildpack-python#1791)

Adds support for the package manager uv:
https://docs.astral.sh/uv/

...which has (very justifiably!) become extremely popular in the last year:

This specifically adds support for uv's project mode (uv sync), which uses a lockfile rather then a requirements.txt file: https://docs.astral.sh/uv/guides/projects/

(While uv does have a pip compatible mode that supports requirements files, using a lockfile is a best practice for deploying an app, and not using one means losing most of the benefits of using uv.)

Apps must have a pyproject.toml, uv.lock and .python-version file (all of which are created by default by uv init followed by either uv lock or any of the other commands that sync/update the lockfile).

We don't support omitting the .python-version file when using uv, since otherwise it leads to a much worse UX in several cases. (For example, when the buildpack default version differs from requires-python in the pyproject.toml file, particularly when requires-python happens to also match the distro Python in the underlying base image.)

The buildpack installs dependencies using uv sync --locked --no-default-groups, to ensure development/test dependencies are not installed.

We don't use --no-dev instead, since that only excludes the dev group, and some apps may be using other group names and have added them to uv's default-groups in pyproject.toml so that local development workflows work with a plain uv sync with no additional args. (PEP 735 doesn't actually prescribe recommended group names, and in fact uses test in its example rather than dev, so non-standard names may be common.)

For more on uv's dependency group behaviour, see:
https://docs.astral.sh/uv/concepts/projects/dependencies/#dependency-groups

uv is installed into a build-only layer, so is not available at run-time (since it's not typically needed at run-time and doing so reduces the image size). Both Python and the entrypoints of installed dependencies are available on PATH, so use of uv run is not required at run-time to use dependencies in the environment.

I've marked this semver: major since in the (probably unlikely) event there are projects using this CNB that have both a uv.lock and also either a requirements.txt or poetry.lock, then this change will cause their builds to error (until one of the files is deleted).

Closes #248.
GUS-W-17141085.

Note: This repository is for the Python CNB (Cloud Native Buildpack) used by Fir-generation apps on Heroku. For uv support for Cedar-Generation apps, see the classic Python buildpack's PR instead.