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.
Here you would need to check projectContext().isActive and if it isn't then return NULL or empty string as appropriate (this might "just work" based on an empty FilePath being returned but I think it's better if the code explicitly communicates that there might not be a project path). Note that I'm going to separately recommend that we use a different approach here so this is just an FYI.