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.
More importantly, they should also get completions when they _don't_ use the pkg:: prefix (since all package functions are in scope). Does this PR handle that as well?