Be able to download the app without running it

afaik I didn't see a way to do everything _but_ `run` without actually running the executable (I want to call `path()`, then call `spawn` aka run the app but with stdio piped etc etc), it'd be cool if there was a `fetch` method that guarantees that `path` will work (I used bin-wrapper to wrap the Google protoc compiler, https://github.com/anaisbetts/protoc-bin)

Read the public run and path entry points and trace how downloading currently occurs. Define fetch's completion by ensuring the executable is downloaded without being run and path() works afterward.

Written by the indexing model from the issue text.