Move 'drupal_core' field alias stuff into prepare_query()

While working on #865510: Add project_solr_browse_projects_form as a block on the main search page when a project type facet is selected we discovered that the "Filter by compatibility" dropdown doesn't work if the project browsing form isn't on the main project download pages. That's because the place where we're doing the field alias isn't being properly reused in all cases. A better place for this would be inside prepare_query(). This has the side benefit of letting prepare_query() work on the project solr stuff if anyone else is trying to use that hook. csevb10 is working on the patch as I write this, so I'm assigning this to him...