plugins always trigger recompilation

When compiling code with a ghc plugin, e.g. ghc -O -fplugin SomePlugin Main.hs, recompilation is triggered for every module. This can make plugins difficult to use in environments with many modules shared between executables.

Since a plugin is a GHC library, I would like to propose that plugin arguments and interface hashes be included in the dependencies for compiled modules, in a similar way to how compiler flags are already included. This would enable ghc to avoid recompilation of modules when using plugins, provided that the plugin and plugin options haven't changed.

Trac metadata

Trac field	Value
Version	7.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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