Port r6062: Tasks for clearing the local cache of artifacts which are known to change upstream occasionally.  #1309.
Port r6062: Tasks for clearing the local cache of artifacts which are known to change upstream occasionally.  #1309.

Also includes a task for clearing any leftover PSC-bundled artifacts from the local repo.  Post-#1310 no more should be created.

git-svn-id: 4b387fe5ada7764508e2ca96c335714e4c1692c6@6070 0d517254-b314-0410-acde-c619094fa49f

NUBIC/psc-mirror,NUBIC/psc-mirror,NUBIC/psc-mirror,NUBIC/psc-mirror