Mark plugins as not buildable if the flag is disabled (#4160)

* Mark plugins as not buildable if the flag is disabled

This ensures that cabal does not consider them at all, and won't try to
solve for their dependencies. So if we turn off the fourmolu plugin, cabal
really won't consider fourmolu at all.

This gets us some of the benefits of #4156 with much less work.

Fixes #4100.

* Stick to no space after flag for consistency