oe.utils.packages_filter_out_system() returns PACKAGES after removing "system"
packages but it doesn't handle ${PN}-src as generated by
PACKAGE_DEBUG_SPLIT_STYLE=debug-with-srcpkg.
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>