Generalize -Xsource to go backwards and forwards, subsume -Xfuture

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let's get rid of -Xfuture

would be great to have e.g., implicit search ignore deprecated implicits when running under a source level that will no longer have these (because the version is one major release past the deprecation version)

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Imported From: https://issues.scala-lang.org/browse/SI-8361?orig=1
Reporter: @adriaanm

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@retronym said:
Reassinging to 2.11.5, 2.11.4 has already been released.

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@som-snytt said:
There is also the -language:future._ movement. scala/scala#5235

Selecting a future source version could select a group of features. Usually the future is uncertain.