Make the key resolver agnostic to different crypto message formats

Different crypto message formats (e.g. OpenPGP/MIME, Inline OpenPGP, etc.)
are irrelevant for finding suitable keys for all recipients. Only the
distiction between OpenPGP and S/MIME (i.e. GpgME::Protocol) is
relevant. Handling different crypto message formats is left to using
code that really needs this (i.e. email applications).

• GnuPG-bug-id: T5283