I am looking for a way to extract the issuer key ID from a signature
file using gpgme without firstly having verified the signature.
Basically, doing something like what gpg --list-packets does.

My software current has a homemade sig file parser that extracts the key
ID from a number of signature files, then it confirms all needed keys
are in the keyring before going onto verify the files.  I'd like to
replace the homemade parser with something more robust.