House all native build output in a per-package dir

In order to ensure there are no stale artifacts as part of a build, this commit
houses all output of native build commands in their own directories. Each
directory is on a per-package basis, and the output is preserved if the package
is fresh or discarded if it is not.

This does not remove the DEPS_DIR environment variable, it just wires it to the
same value as OUT_DIR.