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.
projectName != packageName (the package can be named whatever the user wants and doesn't need to correspond to the .Rproj name or even the containing directory name). In any case you shouldn't need to do this anymore because you've got `packageNameForSourceFile` as described above.