squash! NarSystemMojo: use native-lib-loader when it is a dependency

To support the file layout produced by the NAR plugin, this uses
DefaultJniExtractor#extractJni directly instead of handing off to
NativeLibraryUtil (which would expect the bundled native libraries
in a different path).
squash! Teach NarProperties to extract a list of all known AOLs

Side note: using properties.keySet() does *not* work in NarProperties
because the properties are usually empty. The properties inherit their
defaults from an opaque Properties object passed to the constructor, hence
the keys we are after are only available via the propertyNames() method.