Optionally exclude deprecated elements from root imports

[som-snytt]

A compiler option so that root imports exclude deprecated elements, such as Stream in 2.13.

Root imports are necessarily privileged, as they intrude on all namespaces; this offers stricter quality control.

It would also be useful if the compiler could treat all deprecated elements as though they were already deleted, as the deprecator plugin did, but that is probably out of scope.

[SethTisue]

may run afoul of scala/scala-dev#430

[som-snytt]

Under the -Yimports umbrella, it would be nice (as was proposed) to be able to

-Yimports:scala.{Stream => _, _}

but short of that syntax, a helper option would be -Yimports-no-deprecated.

Any such option is just shorthand for a well-placed compilation unit that hides undesired members of the standard imports and introduces new elements available on a quasi-global basis. But the language mechanics of imports and name binding are unchanged.

[som-snytt]

I see Eugene just asked for this convenience #11317

Someone wanted to hide Stream from standard library on stackoverflow.

[som-snytt]

Closing this enhancement because not a lot of interest in the space of custom context or code prelude.

A REPL could provide these features as a convenience. A codebase can already disable root imports and construct a namespace with ordinary means in the language.