[RFC] Plugin dependencies

Pursuant to https://ghc.haskell.org/trac/ghc/ticket/11244, it would be preferable for users to stop putting plugin dependencies in `build-depends` and a new field.

I propose to introduce a new field, `plugin-depends`, which has the same format as `build-depends`, but adds dependencies on plugins. Packages specified here:
1. Do not get put in the `depends` in the installed package info (so they don't get linked in when you build an executable),
2. Are passed to GHC 8.0 as `-plugin-package-id` (with `-hide-all-plugin-packages`), which forces GHC to only use them to determine scoping of plugin modules, e.g. `-fplugin ModName`). Previous versions of GHC, they get passed in as normal `-package-id` flags for backwards compatibility