This is a partial revert of 6afc718307
to restore the ability to overwrite distutils installed packages.
It is not elegant, but some projects (such as OpenStack's devstack)
rely on overwriting packages installed via the system package manager.
These packages can't be removed because they are dependencies for
parts of the base system, but many of the things devstack needs to run
requires later dependencies. For historical reasons it's not easy to
fix this into a virtualenv, etc, all at once.
If distributions move to setuptools based packages, this problem might
fix itself.
35a3e8b7f5