Include other platforms' libraries in agents

[gabe-sky]

Use Case

I would like to be able to use pdk test unit on any platform, to compile catalogs for any other platform -- even if the providers for the target platform rely on agent-included libraries that are not customarily included with my testing platform's agent.

For example, win32/resolv is not bundled with the Linux agent. And it is required by Windows' host provider. If I try to run an rspec test on Linux, using the on_supported_os pattern and a metadata.json file that indicates compatibility with Windows, rspec fails because it can't find win32/resolv.

Describe the Solution You Would Like

I would like the PDK's version of the Puppet agent to have libraries for all supported platforms, regardless of the platform it's intended to run on.

Describe Alternatives You've Considered

At least for my example above, it's easy enough on Puppet 6 to not fetch the core provider (omit it from my .fixtures.yml file) and then define a non-functional one in my let(:pre_condition) block, so the tests pass.

Additional Context