Use source index to get completions from current package

NOTE: This could use some review as I'm not fully aware of the most 'idiomatic' ways of generating R objects / lists from C++ code.

This PR generates autocompletions from the source index when working on a package project.
Ah but that wouldn't work because the values need to be of different types! (which is why json::Value worked fine). I think it's okay to leave it as is, but honestly I think using the json version was fine (it could also be implemented with a map of string to boost::any or boost::variant but I don't think it's worth the effort right now)