builder: move the gpgkey_type type from Sigchecker to Utils

No functional change, just code motion.
builder: use gpgkey_type for the gpgkey field in sources
builder: pass Sources.source objects directly

Instead of passing the (uri, key, proxy) tuple around, pass the whole
Sources.source record; this requires creating proper Sources.source also
for uri+fingerprint passed via command line.

No functional changes.