Gitea public-only package token access to private OCI blobs

The OCI registry's cross-repository blob mount checked the target package against the token's `public-only` restriction, but checked the source blob only against the full account permissions of the token owner. A `public-only` token with `write:package` scope, whose owner can read a private package and write to a public package namespace, could mount a blob of the private package into the public package. The private blob, such as a container layer, could then be downloaded by anyone through the public package.