Move classes in java.lang to a separate project

Compiling hijacked classes (like java.lang.{Class, Integer, ...})
together with too much other code (for some definition of too much)
can trip scalac pretty badly. This commit changes the build to compile
a minimal subset (all in java.lang) separately to avoid this issue.

As a nice side-effect, it cleans up the directory structure in
the javalib project and removes empty build.sbt files.