Remove `gpg:' prefix from module definitions

Guile's module system is capable of adding prefixes to a module's
exported symbols if a user so desires.  There is no reason to force
these prefixes on users if they aren't necessary.

* src/gpg.scm:
* src/gpg/algorithms.scm:
* src/gpg/errors.scm:
* src/gpg/protocols.scm: remove `gpg:' prefix from symbols