Refactor x509certs::fetchcrl into multiple helper classes.

Break out the body of x509certs::fetchcrl into three internal helper
classes: ::package, ::service, and ::initial, which are 'contain':ed
from x509certs::fetchcrl.  This allows a little bit more abstraction
in dependencies, so we can depend on these classes instead of having
to hardcode the package name everywhere, et.c.

These helper classes are intended to be internal, so end users should
not depend on them directly.

We don't have any immediate use cases for this, but if we are to port
this module to other operating systems than RHEL in the future, it
could be helpful.