Using explicit targets is cleaner and easier to follow.
Also add `forcepush` to pkg and test/pkg and make `push` explicit.
The `dirs` phony was non-existent in all three cases. Likewise `$(DIR)` wasn't
used in that way in pkg or test/pkg and is nolonger used in that way in tools.
Finally, `clean` was bogus in all cases (tools cleaned a thing which never
exists and pkg and test/pkg were a syntax error).
Signed-off-by: Ian Campbell <ijc@docker.com>