pkg_filegroup framework should support templating

In https://github.com/bazelbuild/rules_pkg/pull/273#discussion_r565706282, @aiuto mentioned a shortcoming in the pkg_filegroup framework -- that it doesn't support dynamic naming of files and paths based on attributes defined in `PackageVariablesInfo` providers.

Supporting templating is relatively straightforward, but what isn't necessarily clear is where the templating should go.

In the same thread as above, I [suggested](https://github.com/bazelbuild/rules_pkg/pull/273#discussion_r568126332) that one such option would be to resolve them in the grouping layer (`pkg_filegroup`). However, that may not be the best place. Maybe somewhere in the packaging layer would be better?

Tasks:

- [ ] Determine where templating should be resolved
- [ ] Determine what attributes should be templated
- [ ] Implement and test templating actions