Support lock generation in WORKSPACE

[jvolkman]

This adds pycross_pkg_repo, a repository rule that takes most of the same parameters as pycross_lock_file, pycross_pdm_lock_model, and pycross_poetry_lock_model, and generates a repo containing lock.bzl and all of the packages under //pkg.

The result is that a pycross lock.bzl file no longer needs to be committed/vended in the repository; all one needs is pyproject.toml and poetry.lock or pdm.lock and the lock generation happens automatically, similar to pip_install in rules_python.

This work is mostly a precursor to future bzlmod support.

Example:

load(
    "@jvolkman_rules_pycross//pycross:defs.bzl",
    "pkg_repo_model_pdm",
    "pycross_pkg_repo",
    "pycross_register_for_python_toolchains",
)

# assumes that a rules_python toolchain named "python" has been created.
pycross_register_for_python_toolchains(
    name = "pycross_toolchains",
    python_toolchains_repo_name = "python",
)

load("@pycross_toolchains//:defs.bzl", "environments")

pycross_pkg_repo(
    name = "pdm",
    default_alias_single_version = True,
    target_environments = environments,
    lock_model = pkg_repo_model_pdm(
        lock_file = "@//:pdm.lock",
        project_file = "@//:pyproject.toml",
    ),
)

load("@pdm//:defs.bzl", "install_deps")
install_deps()

and now, bazel build @pdm//:ipython or similar.

See examples/pkg_repo for a complete example.