Fix gensrcs subdirs

The depfile was intended to be placed alongside the per-shard
output file, but was in the final directory instead.  Move
it into the per-shard directory.

Also clean up and comment how shards and shard directories
work.

Test: m framework-cppstream-protos
Test: TestGenSrcs
Change-Id: Ia526ae8997314998814ed8e35598cc2a5bc4aabd