need better support for unpack

Currently when specifiying a dependency set inside an assembly descriptor, if one requires the unpack element to be true, then the unpacking is restricted to the output directory. I have a scenario in which I need to use the unpack element in conjuction with the outputFileNameMapping element.

I would like to request that the outputFileNameMapping be treated as a folder to extract into and appended to the outputDirectory.

So given dependency "foo-1.0.jar", I need a way to extract this into "outputDirectory/foo_1.0/", and since the outputDirectory cannot take properties such as artifactID, and artifactVersion, need a way to plugin this "property support", and making use of the outputFileNameMapping would provide a nice way to handle this.