Add a blacklist of ports that other ports must not depend upon.

Those ports are meta ports, and are only there to improve the user's
experience, in which, if they want, say, python or gcc, they do not have
to figure out what version they should be installing, they simply
install python or gcc.

Reviewed by: antoine
Approved by: adamw
Differential Revision: https://reviews.freebsd.org/D19562