Support cross-building packages

Package managers like `rpm` have considerable support for differentiating between host and target platforms. Most of the time, packages built targeting a platform are built on that platform, e.g. RPM and Debian packages are more often than not built on Linux for Linux.

There are some use cases where developers may want to cross-build packages. This will vary per package manager, but the core of it will require packaging rules to hook into Bazel's platform constraint system.

An example of how to query the constraint system from a rule can be found [here](https://github.com/bazelbuild/proposals/blob/master/designs/2019-11-11-target-platform-constraints.md#proposal-a-check-method-that-takes-a-constraint-value-label).