This commit consolidates all the logic for looking up something in
the Global Reader Environment into the single function lookupGRE.
This allows us to declaratively specify all the different modes of
looking up in the GlobalRdrEnv, and avoids manually passing around
filtering functions as was the case in e.g. the function
GHC.Rename.Env.lookupSubBndrOcc_helper.

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Metric Decrease:
    T8095
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Metric Increase:
    T8095
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