Allow fully qualified paths in {% load %} tag

This removes the requirement of having apps listed in `INSTALLED_APPS` in order for `{% load %}` to find them.

The documentation used to claim that this was due to security reasons but there are better tools to limit available modules (virtualenv).

This change makes it possible to build and distribute tag-only modules without the need of shipping an empty `models.py` and requiring them to be added to `INSTALLED_APPS`.

Ticket: https://code.djangoproject.com/ticket/12772