Add private accessors for goal/sack

For rpm-ostree, I'd like to be able to drive things at a lower level
than the API provided from HifContext.  Using hif-context-private.h
APIs helps me do that.

For example, I want to print a "what packages will be installed" line
between the depsolving and actually running the transaction.

(This patch should help initiate a larger discussion about what
 HifContext should do/expose)