Need DRUPAL_ROOT in file_scan_directory

I have an external PHP script that bootstraps Drupal, and that lives in a subfolder of my Drupal install dir.

After bootstrapping, this script runs some functionality that depends on file_scan_directory() (lives in includes/file.inc). I discovered that this function doesn't currently work if called from a script that lives outside of the Drupal root, as there are several places where it should be (but is not) prepending DRUPAL_ROOT to the file path.

This is a similar bug to #1891802: Need DRUPAL_ROOT when rendering CSS include links

Attached patch fixes this, by doing the following:

- Appends DRUPAL_ROOT to the directory path before calling is_dir() and opendir() (unless it's a stream wrapper URL, or is already an absolute file path)

- Appends DRUPAL_ROOT to the file URI (unless it's a stream wrapper URL, or is already an absolute file path)

- Removes DRUPAL_ROOT from the $file->uri values that get returned by this function, in order to keep this function's return value the same as it is currently (i.e. with values relative to Drupal root).