Rewrite EclipseSystemLayout.getSCLRoots()

- allow SCL without base root "/" (not all SCLs must inherit from base
  system)
- return ordered set of SCL roots (SCL order is signifficant!)
- add sanity checks for contents of $JAVACONFDIRS (it's better to fail
  early that to produce unexpected behaviour)
- implement getSCLRoots(String) to avoid mocking System.getenv()
Use LinkedHashSet/Map instead of HashSet/Map

HashSet and HashMap are unordered.  They can produce non-deterministic
results, are harder to debug and make debugging output appear in wierd
order.  IMHO it is not worth to use them for the little performance
gain they offer over their linked counterparts.
[test] Implement EclipseSystemLayoutTest