make scala-xml a provided dependency for scala-compiler, remove it from scaladoc?

[adriaanm]

scala/scala-module-dependency-sample#14

[lrytz]

WIP PR for Scala at scala/scala#5689 - requires coordination with sbt (scala/scala-module-dependency-sample#14 (comment)).

[dwijnand]

If I understand correctly behind the compiler-interface are compiler-bridges. So we would just need to start shipping compiler-bridges for 2.13+ that used the now-publishing scaladoc jar when compiling xsbt.ScaladocInterface.