Add the ability for extensions to have requirements on other extension

[tfoote]

For example the nvidia extension will need the x11 extension to be useful.

Also the home extension should require the user extension for permissions.

There' s a question of ordering, required before or not. And for each sequential element aka snippets.

Preamble elements are named so order independent, and arguments are order independent as far as I know as long as they're escaped properly.

This will resolve this todo:

rocker/test/test_nvidia.py

Line 134 in cea0680

desired_plugins = ['x11', 'nvidia', 'user'] #TODO(Tfoote) encode the x11 dependency into the plugin and remove from test here

[stonier]

Experimenting with this at stonier#31.

Is it in the vicinity of what you were looking for?

[agyoungs]

This looks stale, but would be pretty useful for a few things I'm working on. Is there any progress on this? I'm happy to take a stab at it if the solution provided by @stonier isn't quite complete and they don't have the time to work on it at the moment.