Allow/disallow pep561 packages

[emmatyping]

@alexander255 has requested a flag --pep561-override to allow listing packages as pep 561 compatible when they are not. This got me thinking and I think the best UX for this would be a pair of flags: --allow-installed-package NAME and --disallow-installed-package NAME (or something like that).

The idea is to solve two problems:

Some people publish PEP 561 packages that are either too unstable or flat out broken for some users. This allows the user to disable the package until things are fixed.

At the other end of the spectrum, some packagers are conservative on adding types, and while they exist, they are not public (yet). Some users would still like to use them.

This would eliminate --no-site-packages, which is a rather blunt instrument.