do_unpack: shift some responsibility around, clean things up

do_unpack: shift some responsibility around, clean things up

- Move the unpacking message printing into do_unpack
- Move the destination directory determination into do_unpack
- Use subprocess's ability to pass in PATH and cwd rather than mangling the cmd
- Use shutil.copy2/copytree for ordinary file "unpack"
- Use the existing urldata from bb.fetch.init rather than re-decodeurl'ing the urls
- Only display the unpack destdir to the user when debugging is enabled, as we
  all know they unpack into WORKDIR

Signed-off-by: Chris Larson <chris_larson@mentor.com>