Build libs without additional profiles, add Enforcer rule

Build libs without additional profiles, add Enforcer rule

Because 'libx' no longer is a submodule of the AspectJ parent POM, it
will not be built automatically each time an AspectJ build runs.
Therefore, is is no longer necessary to shield the zip/unzip steps from
repetitive execution by profiles with auto-activation based on the
(non-)existence of files. An AspectJ developer knows when to build the
module, she does it manually anyway.

A new Enforcer rule makes sure to warn the developer if some files it
expects to exist in the libs folder are not actually present.

Now we also have a Maven Clean rule which wipes away all downloaded and
(un-)zipped files.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>