We need to store the used plugins so that we recompile
a module when a plugin that it uses is recompiled.
However, storing the ModuleNames 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 Trac #15234
We therefor store the ModuleNames of the plugins in a
new field, dep_plgins, which is only used the the