In `oci/private/load.sh.tpl` `tar` is invoked with [process
substitution](https://www.gnu.org/software/bash/manual/html_node/Process-Substitution.html),
which is suppressing it's exit code.

For `tar` errors to be handled, `wait $!` is needed.

As an aside, this class of issues can be linted against with
[`shellcheck` `SC2312`](https://www.shellcheck.net/wiki/SC2312).

`aspect_bazel_lib` and `package_metadata` are not visible from the
generated `oci_pull` repositories. To resolve this the desired symbols
are reexported so `oci_pull` repositories only need to resolve
`rules_oci` which is always guaranteed to work.

closes #575