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.
All of the below (modulo the caching of course which is a good idea) should be replaced with calls to the existing APIs for determining this information). Note that projectDir != packageDir (you can specify an arbitrary subdirectory of a project as the package build directory) -- subtleties like this are why we should use the existing codepaths. I'll describe the right calls to use in a subsequent comment.