Support for transitioning to making all backends opt-in.

Review Request #3972 - Created June 4, 2016 and submitted

Benjy Weinberger
jsirois, patricklaw, stuhood, zundel

Replaces the hard-coded list of backends with an already-deprecated
option. A deprecation warning will issue if that option's values
aren't repeated in --backend-packages.

This allows repos to opt out of currently hard-coded backends,
by removing them from --default-backend-packages. The deprecation
messages encourage repos to explicitly list the backends they
want in --backend-packages, in advance of switching to an entirely
opt-in system.

In short: this change allows repos to opt out, a future change
will require repos to opt in.

Note that the comment about dependencies between backends was bogus.
The fact that the python backend uses the page() target type is
irrelevant to loading the python backend at runtime.

CI passes:

Stu Hood
Benjy Weinberger
Mateo Rodriguez
Benjy Weinberger
Review request changed

Status: Closed (submitted)

Change Summary: