getdeps: Move print "Mapping scratch dir" to stderr
getdeps: Move print "Mapping scratch dir" to stderr

Summary:
Mapping scratch dir <from> -> <to> show up in the output of all the show dir commands on Windows, so removing it.
example: getdeps.py show-inst-dir eden

Reviewed By: wez

Differential Revision: D16092494

fbshipit-source-id: 910288a8d23c1d68c5e70b7b2dbb36ef53a326fc

facebook/wangle,facebook/wangle,facebook/wangle