Refactor plugin portability

[ef4]

This cleans up and simplifies the way we pass babel and htmlbars plugins across processes.

Before we trusted to a lot of fragile code that tried to make our entire objects survive various third-party serialization & deserialization (like, inside babel-loader). Now we always emit json-clean babel config, trusting to our own launcher module to rehydrate parameters as needed.

This was also necessary to make room to implement the pluginLocationHints feature, which lets you avoid the penalty of non-serializable babel configs even if not all your upstream dependencies are playing nice, and will be in the next PR.