implicitRequirementsShallow can never reach HoleUnit

Rodrigo Mesquitarequested to merge
wip/romes/27053 into master 
implicitRequirementsShallow can never reach HoleUnit

    findImportedModule will never return `HoleUnit` for a `ModuleName`
    (a `HoleUnit` can only be found as a signature instantiation, never as a
    directly *imported* thing)

    Therefore, we can drop `[ModuleName]` returned by
    `implicitRequirementsShallow`, which makes many things dead code.
    Namely, the call to `implicitRequirementsShallow` from
    GHC.Driver.Downsweep which was a performance bottleneck (for doing lots
    of duplicate work in findImportedModule) is now entirely gone.

Fixes #27053 (closed)

Edited by Rodrigo Mesquita

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