instead of using the old baseName=smack appendix=project.name approach,
we are now going convention over configuration and renaming the
subprojects directories to the proper name.
Having a prefix is actually very helpful, because the resulting
libraries will be named like the subproject. And a core-4.0.0-rc1.jar is
not as explicit about what it actually *is* as a
smack-core-4.0.0-rc1.jar.
SMACK-265
This allows us to exploid Java 8 streams and Java 5 for-each
loops. The returned Collections are usually unmodifiable. We decided
against returning Iterable because this would mean determining the
size in O(n) compared to Collection.size() which is often faster
(e.g. O(1)).