Fix #856: Make javalibEx compile and properly test in CI
Merge pull request #857 from gzm0/fix-javalib-ex

Fix #856: Make javalibEx compile and properly test in CI
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.
Merge pull request #859 from gzm0/split-javalib

Move classes in java.lang to a separate project
Intern scalajsenv.js via a source generator.

We fully remove the tool's dependency on I/O and Java-style resources
by generating a .scala source file that contains the whole
scalajsenv.js as a literal string.