dev-python/pypy: Apply full Gentoo path set, only for Gentoo sys.prefix

dev-python/pypy: Apply full Gentoo path set, only for Gentoo sys.prefix

Override all default distutils.command.install paths for Gentoo rather
than just scriptdir. However, do that only when install_base equals
the default prefix in which PyPy is installed system-wide.

This fixes both issues resulting from lack of prefix override (like
installing data files in /usr/lib*/pypy), and from overeager prefix
overrides (like virtualenv installing scripts in /usr/bin).

Fixes: https://bugs.gentoo.org/462306
Fixes: https://bugs.gentoo.org/465546