1. Refactors the duplicated `buildExtraSources` function from `gbuild` and
    `buildOrReplLib` into a standalone monadic computation in the context of
    building a component. This refactor allows
    us to share the code for building an extra source amongst the two
    functions.

2. Creates a new module Distribution.Simple.GHC.Build.Modules which, in the
    same spirit as ...GHC.Build.ExtraModules, defines an action which builds
    all the Haskell modules of the component being built.

    This function clarifies and re-implements the logic of building Haskell
    modules in the different possible ways, while accounting for
    Template Haskell special "way requirements", which was previously
    duplicated in a non-obvious manner in gbuild and buildOrReplLib.

    The Note [Building Haskell modules accounting for TH] in that module
    explains the big picture, and the implementation is re-done in light of
    it.

3. Re-work the linker invocations, focusing on preserving existing
behaviour before simplifying or fixing bugs any further.

Fixes #9389.