Moves AbstractFileClassLoader to scala-reflect.jar

Its string name was used in ReflectionUtils and became broken after repl
got factored out. This hints that that classloader belongs to where it is
used, i.e. to scala-reflect.jar.

Moreover AbstractFile is defined in scala-reflect.jar, so it's only
logical to also define a derived classloader in scala-reflect.jar.
Merge pull request #2568 from scalamacros/topic/abstractfile-classloader

Moves AbstractFileClassLoader to scala-reflect.jar