Teach rpm about pre-transaction dependencies

Teach rpm about pre-transaction dependencies
- A pre-transaction dependency is generally anything that must be
  available at the start of the transaction, and cannot be resolved
  by packages *in* the transaction. This lets %pretrans scriptlet
  dependencies be expressed correctly, and could be also used for
  other kinds of pre-conditions.
- rpmlib() dependencies are a special case of pre-trans dependencies
  but leaving them handled separately as they cannot be provided by
  anything in rpmdb either, whereas pretrans dependencies can.