packagegroup: Handle SPDX signature issues

Packagegroups have "special" sstate signature behaviour in that they don't rebuild
when their dependencies do. This causes problems for SPDX task signatures. Since
packagegroups don't actually have any code in them, we don't need the standard
dependencies anyway so cancel out and allow the sstate signatures to function
correctly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>