oci: support --only=package (#45775)

Previously `spack buildcache push --only=package` errored in the OCI
case, but it's been requested that OCI can be used as pure storage w/o
the need for runnable container images.

This commit makes it so that

1. manifests refer only to runtime dependencies that were selected to be
   pushed
2. failure to upload a blob among the selected specs does not prevent a
   manifest/tag to be created for dependents: they just don't refer to
   the missing blob as a layer/dependency

This fixes the following issues:

1. dependents of non-redistributable specs can now be pushed to oci
   build caches without error
2. failure to upload one tarball does not cause cascading failures for
   dependents whose tarballs do upload succesfully -- so it's better
   best-effort behavior
3. for some people uploading with deps caused a massive amount of
   fetches of their manifests (which certain registries count as a
   download of an image, even though their layers are not fetched) --
   being able to specify --only=package reduces the number of fetches.

0 files changed, 0 insertions, 0 deletions