rules_pkg outputs should be usable as runfiles

[nacl]

In #160, it was discovered that it is not possible to use the outputs of the experimentalpkg_rpm rule as runfiles inputs for other rules, quite possibly due to not returning aDefaultInfo 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.