tweak deprecatedInheritance to turn a blind eye to same-compilation-unit infractions

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(No description for SI-7312.)

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Imported From: https://issues.scala-lang.org/browse/SI-7312?orig=1
Reporter: @retronym

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@retronym said:
As discussed: https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/aFgC0rY_kFg

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@retronym said:
scala/scala#2336