Remove packages which are no longer valid

 * Before we start a build we check the SYSROOT for old packages which no
   longer have a valid package definition.  This could be the result of a
   rename, or simply removing the package all together.  Regardless, we remove
   it from the SYSROOT.
Fixup clean/distclean

 * Clean tmp files for a package out of BUILDER_TMPDIR during clean/distclean.
   This is particularly relivant after we added prepped/compiled staging data
   to temp.