Make django_orm load virtualenv packages before system packages

A django app may require a specific version of a package that it
depends on, and will have ensured the correct version is in the
virtualenv.

This commit ensures that this version will be used, even if a
different version is installed in the system python.

See issue #8024
Merge pull request #8025 from davidn/develop

Make django_orm load virtualenv packages before system packages