Followup from #7269: Add .php extension to PHP files
Having .module in module files, .profile in profile files etc makes everything harder to read and once the issues below are in, we will not need them to figure out what anything is, in drupal_system_listing and friends.
Postponed on #1292284: Require 'type' key in .info files
Related issues
#340723: Make modules only require .info files
#1033116: Rename template.php to THEMENAME.theme to elimnate ambiguity around "the template file" vs. "template files"