Inconsistent "pipeline" module names in 8.12

I hope this is not too late in the relase cycle: I realised that we have a bit of an inconsistency in module names for modules that define optimisation phases for the differnet IRs:

Core: GHC.Core.Opt.Driver, includes non-optional passes like CorePrep
STG: GHC.STG.Pipeline
Cmm: GHC.Cmm.Pipeline

All "pipelines" are actually non-optional, so Core is the outlier here. I suggest moving it to GHC.Core.Pipeline. Thoughts? @hsyl20 @bgamari @simonpj

Edited Jul 27, 2020 by Sebastian Graf

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