getdeps: fix builds with cached project downloads
getdeps: fix builds with cached project downloads

Summary:
D21364132 accidentally broke this; we can't run the fetcher
for projects for which we pulled the build out of cache, because there
is no source to update in that case.

This commit adjusts the logic so that we write out a marker file
to indicate that we installed a build from cache and to look for
that file being present to gate the new update logic.

Reviewed By: lnicco

Differential Revision: D21419122

fbshipit-source-id: 304670848add22531d88549d66f22c40ff255140

facebook/wangle,facebook/wangle,facebook/wangle