Split rpmbuild dependencies to separate _rpmb module

Split rpmbuild dependencies to separate _rpmb module
- installers and the like dont need the pile of poo called librpmbuild for
  anything, avoid dragging it in needlessly
- import _rpmb into rpm namespace if available but dont complain if
  its not there
- arrange ts.parseSpec() compatibility on python side by dynamically
  importing _rpmb if possible