simplify and normalize the compliance-check phase

[ddyer0]

The current compliance check uses an ad-hoc search to find sources newer than the previous compliance-check,
as an optimization to skip redundant check. This ad-hoc knowlege of the source hierarchy doesn't mesh with the
rest of maven, and is incompatible with using maven to structure projects except exaxtly as codename1 currently
does it.

I propose that compliance-check should instead check the dates of the .classes that are actually going to
be checked. This would be simpler, and also compatible with other project structures.

I note that the proguard phase already works this way.