[doctest] Allow filtering which py files are considered for --doctest-modules

When doctestmodules is True, we unconditionally attempt to doctest all files, except for a few explicitly denylisted file names:

    if file_path.suffix == ".py":
        if config.option.doctestmodules and not any(
            (_is_setup_py(file_path), _is_main_py(file_path))
        ):
            mod: DoctestModule = DoctestModule.from_parent(parent, path=file_path)
            return mod

In some situations, such as happened in https://github.com/pytest-dev/pytest/issues/1881, it would be useful to limit the set of files to be doctested; e.g., a fairly common desire is to only doctest files in src/, and not bother trying to doctest test files. In my situation, I have a repository that has a number of scripts that do not have __name__ == '__main__' guards, and doctest immediately chokes on them.

The fix is relatively simple, subject to bikeshedding how exactly the option should be spelled. I would propose we name the new option --doctest-modules-glob, and have it work equivalently to --doctest-glob but generating DoctestModule instead of DoctestTextFile. I can submit the PR, if someone agrees to help shepherd it. Thanks!

(This is technically a dupe of https://github.com/pytest-dev/pytest/issues/1881 but I am submitting a fresh issue because there's a lot of irrelevant noise in the original issue.)

The relevant entry point is the --doctest-modules collection branch shown in the issue; compare its file matching with --doctest-glob. Add coverage for selecting module files while excluding nonmatching files, and verify that the existing setup.py and main.py exclusions still hold.