Update Set hack to BuildAction.
Update per new Coopr API.

 * r8214: https://software.sandia.gov/trac/coopr/changeset/8214

Coopr never deprecated the 'rule=' keyword to the Set() constructor, but
recently removed it (r8214) in favor of the 'initialize=' keyword.  The
change is semantically for the better; it just requires a minor change
in our code.
Remove unused import.