Allow registering (Source)Plugins through the GHC API

One might expect to be able to load plugins through the GHC API by doing something like:

runGHC $ do
  dflags <- getSessionDynFlags
  setSessionDynFlags dflags { plugins = [LoadedPlugin some_plugin _conjure_up_modiface []] }

but this doesn't actually work because the plugins field in DynFlags is used as a cache and overwritten by initializePlugins whenever plugins loaded through the command-line need reloading. Not to mention that there isn't a meaningful way to fill the _conjure_up_modiface hole AFAIK.

While in principle it might be possible to use source plugins via the API right now by messing with the exposed cmdline flags the right way it feels much cleaner to just have a new type of plugin that can be added to GHC right through the API.

Trac metadata

Trac field	Value
Version	8.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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