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.
We already have a way to do this. I would have this function be renamed "getCompletionsActivePackage" (or something like that) and have it take the package name as a parameter. You can get the package name from the filePath by calling `module_context::packageNameForSourceFile`.