Splitting The World: Python packaging and supporting multiple backends

[pathunstrom]

Constraints:

pip install ppb needs to produce the default desktop implementation as it exists now.

When installing for a web context, we don't want SDL included, so need some method to get "core" "features" and "ppb-web-backend" (We can shave the naming yak later).

We should start having a conversation on what this looks like and how to handle it overall.

Current options are deep magics to create a web distributable or ppb package explosion with explicit dependencies. (Or the worst of both worlds: package explosion with deep magics!)