openpgp: 'lookup-key-by-{id,fingerprint}' return the key first.

openpgp: 'lookup-key-by-{id,fingerprint}' return the key first.

Previously, 'lookup-key-by-{id,fingerprint}' would always return the
list of packets where the primary key is first.  Thus, the caller would
need to use 'find' to actually find the requested key.

* guix/openpgp.scm (keyring-insert): Always add KEY to PACKETS.
(lookup-key-by-id, lookup-key-by-fingerprint): Change to return the key
as the first value.
(verify-openpgp-signature): Remove now unneeded call to 'find'.
* tests/openpgp.scm ("get-openpgp-keyring"): Adjust accordingly.