Fix issue #175. This is a bit of a one-off hack for find-symbol-at-point
only but might be enough.

We will try the query with another projects if it matches certain
criteria.

       - We didn't find anything with the current project
       - The path in question (likely a header) does not start with the current
         project's path (there's room for mistakes here with symlinks).
       - The file in question does start with another project's path
       - The other project is loaded (we will start loading it if it's not)