Split up -sourcepath usage between scaladoc and -Y option for bootstrapping

[scabug]

-sourcepath should be a -Y setting, it's only used for compiling the standard library.

A separate flag should be added for Scaladoc, to support -doc-source-url.

Apply the same change to Scala 3.

https://github.com/sbt/zinc/issues/1304#issuecomment-1877155832

Older discussion https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/8xbRfONJL7U.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7125?orig=1 Reporter: @VladUreche Affected Versions: 2.11.0-M1

[lrytz]

@SethTisue agree with the proposed cleanup here? It should be coordinated with Scala 3. (I updated the issue description).

[SethTisue]

I haven't encountered -sourcepath before, either personally or in online discussions. But I read your comment (https://github.com/sbt/zinc/issues/1304#issuecomment-1877155832) and I don't see any reason to object.