Toolchain cache key should also include plugin-detected version files

[Phault]

Most of proto's plugins also tries to resolve the tool version from additional first-party sources such as package.json, .nvmrc, rust-toolchain.toml, etc. In case the resolved version stems from of these files, the file should also be included in the final cache hash.

The easiest route would be adding an additional input for specifying additional sources manually. But it would better if the actually used sources could be retrieved from proto itself, so it just worked automagically.

[milesj]

This is a bit tricky, since we don't know what those version files are without loading and executing every WASM plugin.

I'll see if there's a way to dump this info, but i think that would still happen after the initial key is already hashed.