Shift (sub)key -> public key coercion into _serialize_signature_packet_target

While RFC 4880 explicitly specifies that key fingerprints are
calculated only on the public part of the key packet, it was unclear
if signatures are also only calculated over the public part.  It turns
out that they are, so move the coercion logic from
PGPPacket._parse_generic_public_key_packet into
_serialize_signature_packet_target.