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.
I think that after looking for package completions in the actively developed package we should check to see whether this source file belongs to another package and then look in that package's namespace (you can do this using `module_context::packageNameForSourceFile`. This will be helpful for cases where users are working on a package outside of the context of an RStudio project.