implement a story for sharing bosatsu files without a build tool like bazel

[johnynek]

One idea would be to have a notion of dependencies, then we could have a command to fetch the dependencies, compile them, and write them into a single protobuf file.

We could set up dependencies such as:

[
  {
     "package": "Package/Name/Here",
     "hash_algo": "sha256",
     "hash_value": "f34f3434...",
     "urls": ["http://foo", "http://bar"]
  },

]

we could potentially support both compiled protobuf or source code that also has some description of what compilation flags are needed (e.g. package_roots...). Accepting source is nice because we can share easily via github or any source tracking system without committing binary artifacts.

Source sharing interacts well with #414