Add `python_visibility` gazelle directive

[dougthor42]

🚀 feature request

Relevant Rules

• gazelle

Description

Summary: Add a py(thon)_visibility gazelle directive that acts like the go_visibility directive. This directive would append additional labels to the visibility attribute on generated targets.

Example:

# gazelle:python_visibility //tests:__pkg__
# gazelle:python_visibility //bar:baz

py_library(
    name = "directive_python_visibility",
    srcs = ["foo.py"],
    visibility = [
        "//:__subpackages__",
        "//bar:baz",
        "//tests:__pkg__",
    ],
)

Describe the solution you'd like

I have a branch that does this already. I'm mostly making this Issue for discussion before I submit the PR.

Branch: https://github.com/dougthor42/rules_python/tree/gazelle-python-visibility

Describe alternatives you've considered

Manually adding other vis labels is the only other option I've found. That's not really an option when adding gazelle to an existing, large project.