Offer a weaker name shadowing warning

Suppose I have a module

module Survive where
import HuntGather

flounder = goFishing river
  where
    goFishing = ...

If the maintainer of HuntGather decides to add a goFishing function, my module will suddenly get a name shadowing warning, although there is little risk of confusion. So I think it makes a lot of sense to offer, at least, a version that ignores shadowing of *implicitly imported* names, and to use this for -Wall instead of the current version. Another step down would be to ignore shadowing of all top-level names, but having three levels of name shadowing warning would be too much trouble by far.

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Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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