Plugin dependency information is stored separately

We need to store the used plugins so that we recompile
a module when a plugin that it uses is recompiled.

However, storing the `ModuleName`s of the plugins used by a
module in the `dep_mods` field made the rest of GHC think
that they belong in the HPT, causing at least the issues
reported in #15234

We therefor store the `ModuleName`s of the plugins in a
new field, `dep_plgins`, which is only used the the
recompilation logic.

Reviewers: mpickering, bgamari

Reviewed By: mpickering, bgamari

Subscribers: alpmestan, rwbarton, thomie, carter

GHC Trac Issues: #15234

Differential Revision: https://phabricator.haskell.org/D4937

(cherry picked from commit 52065e95)