this is going to be a serious version of //tools/depot-build.
right now it doesn't support parsing any target specs yet, so only
shells and builds for the physical project of the current folder work.
Change-Id: I4308e29da940571622ff9e539fbb8ededd27aca7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4335
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>
