gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH.

Using PYTHONPATH as a mean to discover the Python packages had the following
issues:

        1. It is not versioned, so different versions of Python would clash if
        installed in a shared profile.

        2. It would interfere with the host Python site on foreign
        distributions, sometimes preventing a a user to login their GDM
        session (!).

        3. It would take precedence over user installed Python packages
        installed through pip.

        4. It would leak into Python virtualenvs, which are supposed to create
        isolated Python environments.

This changes fixes the above issues by making use of a sitecustomize.py
module.  The newly introduced GUIX_PYTHONPATH environment variable is read
from the environment, filtered for the current Python version of the
interpreter, and spliced in 'sys.path' just before Python's own site location,
which provides the expected behavior.

* gnu/packages/aux-files/python/sitecustomize.py: New file.
* Makefile.am: Register it.
* gnu/packages/python.scm (customize-site)
(guix-pythonpath-search-path): New procedures.
(python-2.7)[phases]{install-sitecustomize.py}: New phase.
[native-inputs]{sitecustomize.py}: New input.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
(python-3.9)[native-search-paths]: Likewise.
[phases]{install-sitecustomize}: Override with correct version.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
* gnu/packages/commencement.scm (python-boot0):
[phases]{install-sitecustomize}: Likewise.
[native-inputs]{sitecustomize.py}: New input.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
* guix/build/python-build-system.scm (site-packages): Do not add a trailing
'/'.

squash! gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH.

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