www/py-django-mptt: Fix Django dependency causing conflicts

Tweak RUN_DEPENDS, switching to file-based django detection, instead of
by package name.

This ensures that if a port that depends on this one, also depends on a
different django version, it will see and use the installed version,
rather than attempting to install another one, which results in a
conflict as all django ports conflict with each other.

www/py-django-dpaste depends on py-django-mptt but is about to be
limited to Django <= 1.6 (it does not currently support higher).

Since py-django-mptt depends on www/py-django (currently 1.8.x), this
results in the conflict described above.

The same change will also be made in www/py-django-dpaste for any ports
that may depend on *it* in the future.

This change is a NOOP for other conditions.

See r390184 [1] for another instance of this issue.

[1] https://svnweb.freebsd.org/changeset/ports/390184

Approved by: portmgr (blanket: trivial, tested, build fixes)