doebuild: avoid redundant distfiles checks

doebuild: avoid redundant distfiles checks

When the unpack phase is already marked as complete, it's wasteful to
check distfiles digests. In order to avoid this, we have to migrate the
distfiles/workdir timestamp comparisons from ebuild.sh to doebuild.py,
so that doebuild always knows when unpack will be triggered. This also
allows us to eliminate code in dyn_unpack that duplicated dyn_clean,
actually call dyn_clean instead.