This adds a new entry_point_dependencies field to python_tests and python_test targets.

This allows tests to depend on a subset (or all) of the entry_points defined on python_distribution targets (only on the entry_points field, not in the provides field).

For example:

python_tests(
    name="tests",
    entry_point_dependencies={
        "//address/to:python_distro_tgt_1": ["*"],  # all entry points
        # only one group of entry points
        "//address/to:python_distro_tgt_2": ["console_scripts"],
        "//address/to:python_distro_tgt_4": ["st2common.runners.runner"],
        # or multiple groups of entry points
        "//address/to:python_distro_tgt_5": ["console_scripts", "st2common.runners.runner"],
        # or 1+ individual entry points
        "//address/to:python_distro_tgt_6": ["console_scripts/foo-bar"],
        "//address/to:python_distro_tgt_7": ["console_scripts/foo-bar", "console_scripts/baz"],
        "//address/to:python_distro_tgt_8": ["st2common.runners.runner/action-chain", "console_scripts/foo-bar"],
    },
)

A dependency defined in entry_point_dependencies emulates an editable install of those python_distribution targets. Instead of including all of the python_distribution's sources, only the specified entry points are made available. The entry_points metadata is also installed in the pytest sandbox so that tests (or the code under test) can load that metadata via pkg_resources or importlib.metadata.entry_points or similar.

Misc notes:

• I added this to the pants.backend.experimental.python backend and also registered the rules I extracted from pants.backend.experimental.python.framework.stevedore with the stevedore backend since it needs those rules.
• Unlike all other dependencies fields, entry_point_dependencies is a dict[str, list[str]]. The .keys() of the dict are logically similar to standard dependencies fields. Using a dict provides more granular control over the dependencies.
• Also unlike other dependencies fields, this field does NOT result in a dependency on the python_distribution target. Instead, that target provides the entry_points metadata to infer dependencies on the actual code for those entry points.
• I extracted most of the logic for this from the experimental stevedore plugin. Enabling the stevedore plugin is not required to use this feature.

Closes #11386
Closes #15481