xautoload.module sometimes not included from xautoload.install

Normally when Drupal loads a module for installation, it will first include the *.module file and then the *.install file.
xautoload needs this, because xautoload.module activates the class loading stuff.

However, in some cases it can happen that Drupal fails to include xautoload.module before the *.module.
This happens if
- you run drush en -y MODULENAME
- MODULENAME depends on xautoload.
- xautoload is automatically downloaded and installed.

This is explained in more detail here:
#2228825: drupal_get_filename() does not search the filesystem

As a workaround, we should simply include xautoload.module explicitly from xautoload.install.