Improve pip package determination (#63321)

Improve pip package determination (#63321)

Summary:
Invoking `pip` or `pip3` yields list of packages invoked for `pip` alias on the path, rather than for the one currently being executed. Changed `get_pip_packages` to use `sys.executable + '-mpip'`

Also, add mypy to the list of packages of interest

Discovered while looking at https://github.com/pytorch/pytorch/issues/63279

Pull Request resolved: https://github.com/pytorch/pytorch/pull/63321

Reviewed By: walterddr

Differential Revision: D30342099

Pulled By: malfet

fbshipit-source-id: fc8d17cf2ddcf18236cfde5c1b9edb4e72804ee0