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 we should be calling a C++ shim function which calls `module_context::isRScriptInPackageBuildTarget` (note that that function should call `module_context::resolveAliasedPath` on the passed path as it might have a ~ in it). When you get the results from this query back you should only include functions that are in the _same_ directory as the filePath (as you will also be getting source index results from test, inst, etc.).