rules_pkg outputs should be usable as runfiles

In #160, it was discovered that it is not possible to use the outputs of the experimental`pkg_rpm` rule as runfiles inputs for other rules, quite possibly due to not returning a`DefaultInfo` provider. Given they are simple binary blobs, packaging outputs should be available for consumption in other rules this way.

This may apply to the other rules in rules_pkg too, and should be investigated and rectified if runfiles are found not to be supported.