Turn implementation restriction for SI-5954 from 2.10.x into a warning

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The implementation restriction created from #5954 has two problems. 1) The problematic code works fine if compile with sbt. That means the restriction is breaking some people needlessly. 2) It's not binary compatible. The implementation restriction needs to be turned into a warning and the setting to get around the restriction needs to be removed.

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Imported From: https://issues.scala-lang.org/browse/SI-7070?orig=1
Reporter: @JamesIry
Assignee: @JamesIry
See #5954

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@JamesIry said:
scala/scala#2079

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@adriaanm said:
in principle we don't introduce new warnings/errors/different type inference/different implicit resolution in minor releases, but I feel there's a case to be made here of how broken package objects are that we should warn