chore: use language import instead of -Ycompile-scala2-library

[hamzaremmal]

This PR adds a new language import (scala.language.2) to replace the -Ycompile-scala2-library flag.
This language import should only be used in the stdlib (package scala). To enforce this, the language import is private[scala] but we still need to enforce it (#23081).

For now, I've allowed the -source 2 option but this will be disallowed once we add the source files of the stdlib in this repository.

[hamzaremmal]

Since scala.language.2 lives in the patches, we will need to patch the denotations to gain access to it. Otherwise, typer complains because the member is not found.

[WojciechMazur]

Wouldn't this one leak into the -source setting choices that can be specified by users?

I guess that the main goal of this PR is to restrict access to special mode of compilation Scala 2 standard library so that it cannot be used in any other project. Is there a plan to compile any other sources then Scala 2 stdlib? If no, then maybe special marker class added to sources would be enough to to check at compile time (eg. using requiredClass method) that -Ycompile-scala2-library is not used in foreign scope.

[hamzaremmal]

Wouldn't this one leak into the -source setting choices that can be specified by users?

As it is right now, yes. And I actually use it to make this work until I add the imports manually. But when this is done, it will be forbidden in -source. We can disallow the option by adding "2" here:

scala3/compiler/src/dotty/tools/dotc/config/SourceVersion.scala

Line 46 in 15f40a9

val illegalSourceVersionNames = List("3.1-migration", "never").map(_.toTermName)

[hamzaremmal]

I guess that the main goal of this PR is to restrict access to special mode of compilation Scala 2 standard library so that it cannot be used in any other project.

Actually, this is not the primary goal of this PR. We will be adding the source files of the scala2 library in the same folder as the scala3 library and we need a way to still behave as scala2 for those specific files. A flag affects all the files while an import can affect on specific compilation units.

[WojciechMazur]

Aren't all files of single sbt project part of single compilation unit beside macros? I always thought there is no effective difference between the flag and import, but maybe I've missed something

[hamzaremmal]

We will revert commit: 43e45e7 when we merge this PR

[hamzaremmal]

I'm renaming the object from 2 to 2.13