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.
This is a setting used for the old R autocompletion engine, and we no longer use that anywhere in our completions (I believe this was a 'hack' to get file completions turned on; we now have our own (better) completions, e.g. avoiding doubly-inserted `//` and such)