When python_generation_mode is project or file , the generated py_test targets are consistent with py_library. However, when python_generation_mode is package, it becomes inconsistent: it requires either __test__ target or __test__.py file to generate py_test in package mode, otherwise it will fall back to file mode.

This PR relaxes this requirement with a new directive gazelle:python_generation_mode_per_package_require_test_entry_point. Whent it's set to false, Gazelle and generates one py_test target per package in package mode even without entry points.

This allows people to use gazelle:map_kind to map py_test to a macro that sets a default test runner, such as rules_python_pytest or pytest-bazel, and generate one test target per package. The behavior when gazelle:python_generation_mode is "file" or "project" remains the same.

This fixes #1972 for supporting pytest from Gazelle. With this approach, people can define a thin macro like this to use py_pytest_main:

load("@aspect_rules_py//py:defs.bzl", "py_pytest_main")

def py_test(name, **kwargs):
    py_pytest_main(
        name = "__test__",
        deps = ["@pip_pytest//:pkg"],  # change this to the pytest target in your repo.
    )

    deps = kwargs.pop("deps", [])
    deps.append(":__test__")

    py_test(
        name = name,
        main = ":__test__.py",
        deps = deps,
        **kwargs,
)

BREAKING CHANGES:
Without gazelle:map_kind or __test__ target or __test__.py, the package mode will now generate py_test without main attribute, which may not be runnable. However, this is already an issue with "python_generation_mode:project" before this PR.

The default value of gazelle:python_generation_mode_per_package_require_test_entry_point is true to preserve the current behavior. We will flip that default value in the future.