Restructure how Policy plugins work

[justinvp]

From Joe:

I am only now realizing policy plugins don't work the way I had thought.

Instead of having a plugin per policy pack, we have a single universal plugin, pulumi-analyzer-policy, which loads Node.js program that dynamically loads the desired policy pack.

This seems like it will fundamentally make it difficult to ever support more than just Node.js.

The alternative, which I assumed we were doing, was to use the plugin system to distribute, install, and manage policy packs, and to use the ability to have many versions side-by-side. That would get us multi-language while also aligning with how the rest of the system works.

This is the underlying work to enable other language SDKs and support OPA.