Support detecting multiple site-packages directories in venvs

[truls]

On Red Hat derived systems, Python's purelib and platlib are set to <prefix>/lib and <prefix>/lib64 respectively. This means that Python packages are installed to either of those directories depending on whether they contain native libraries or not (as I understand it). This distinction is also present in pipenv's virtualenvs. See:

$  . /home/truls/.local/share/virtualenvs/pyright-test-Fm_xQ9MX/bin/activate
(pyright-test) truls@dhcp-111-236 ~/foo/pyright-test $ python
Python 3.9.0 (default, Oct  6 2020, 00:00:00) 
[GCC 10.2.1 20200826 (Red Hat 10.2.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sysconfig
>>> sysconfig.get_path('purelib')
'/home/truls/.local/share/virtualenvs/pyright-test-Fm_xQ9MX/lib/python3.9/site-packages'
>>> sysconfig.get_path('platlib')
'/home/truls/.local/share/virtualenvs/pyright-test-Fm_xQ9MX/lib64/python3.9/site-packages'
>>> 

The current virtualenv include path detection logic in pyright returns only the first extant site-packages directory it finds. This is insufficient for discovering the full include path for virtualenvs where packages may be installed in both the lib and lib64 directories. This PR changes the virtualenv include path detection logic so that it returns all of the extant site-packages dirs it finds in the lib, lib64 or Lib directories.

[ghost]

All CLA requirements met.

[erictraut]

Thanks for the contribution!