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.
The right way to do this is have the RCompletionManager take the HelpStrategy as a parameter to it's initialize method (we don't pass global objects rather we get them injected into the constructor or the initialize method)