Spin off xml library as scala-library-xml.

1. wean off xml in core sources (those built by locker: library/reflect/compiler)
2. make compiler flexible: 
    - don't require scala.xml unless xml literals were parsed
    - when Predef.$scope does not exist, but scala.xml.TopScope does, alias the latter to $scope (our old xml library assumes $scope is always in scope)
3. spin off scala.xml.* to src/library-xml and scala-library-xml.jar, and remove Predef.$scope (the last dependency of core sources on xml)

review by @retronym, @paulp, and anyone else who'd like to dance on scala.xml's shallow grave.