Fixes #3354

This adds a new Gazelle directive python_generate_pyi_srcs
which accepts a boolean value true or false. It defaults
to false for backwards compatibility.

When true, the directive causes .pyi files, whose name matches
with a .py file found in srcs, to be added to the pyi_srcs
target attribute.

This helps with cases where manually-generated .pyi files are
present in the project and are needed for things such as rules_mypy.

Given the following files in a my_dir package:

BUILD.bazel
__init__.py
a.py
a.pyi
b.py
c.py
c.pyi

# gazelle:python_generate_pyi_srcs true will cause Gazelle to
generate:

py_library(
    name = "my_dir",
    srcs = [
        "__init__.py",
        "a.py",
        "b.py",
        "c.py",
    ],
    pyi_srcs = [
        "a.py",
        "c.py",
    ],
)