Move scalaInstanceTopLoader to compileBase settings

[adpi2]

• Move scalaInstanceTopLoader and useScalaReplJLine to compileBase:
  They are/should only be used in the JvmPlugin to create the scalaInstance.

• Move jansiExclusionLoader (for Scala 2.10 only) to the scalaInstanceTopLoader implementation

• Remove the ClassLoaderCache hack for dotty:
  It is not needed anymore, since dotty uses sbt 1.5

• Make Defaults.makeScalaInstance public:
  ScalaInstance and the scalaInstance task are public. But, to create a custom scalaInstance we need the extendedClassLoaderCache which is package private. That's why I think makeScalaInstance should be public.

This will allow us to properly bootstrap the Scala compiler in the dotty repository. See scala/scala3#12259.

[eed3si9n]

I haven't branched out develop to 1.5.x yet. Is this more of a 1.6.0-M1 feature or can this be "backported" to 1.5.x without going through the RC cycle?

[adpi2]

It is not forward compatible because I changed the scope of scalaInstanceTopLoader. So it should go to 1.6.x.

I can backport a lighter version, that just makes makeScalaInstance public.

[adpi2]

Maybe we can merge #6481 and put this one on hold for now.

[adpi2]

@eed3si9n This one should be ready to merge.