Use a unique depfile for each gensrcs command

gensrcs modules run the same command once for each input file to
produce each output file.  Each command needs its own depfile instead
of reusing one per shard.  The depfiles will be merged together
by RuleBuilder into one depfile per shard.

Test: TestGenSrcs
Change-Id: Iaf4f2cf9f5592c20e32944ddf34e0a61aff17ba8