DX: Make file_scan_directory() use save property names as file_load()

Issue Summary

This issue is based on the following request on Drupal core improvements » File system:
“file_scan_directory() should return objects that have the same properties as the $file object returned by file_load(). Right now the file path is $file->path instead of $file->filepath, base name is $file->basename instead of $file->filename. The main complication of this patch is that there are quite a few core functions that directly return the results of this function so the change cascades out.”

I've made a patch that changes this, however, it doesn't go all the way and enforce this terminology throughout Drupal. E.g. it does not rename {registry}.filename to {registry}.filepath even though the fields contains a path a not just a path-less filename. Do we want to do that? And do we want to do it in one big issue, or is this better handled in separate issues?