Use this method to indicate when some other files (from the same
package) are needed to be sourced before the current file. This
method is useful in cases where the alphabetical loading order is
a problem.
A typical use-case is a file defining a subclass of another
class. In this case, the file defining the subclass will require
the definition of the base class.
Source files extending classes of the current file.
When classes are defined in this file and (some) of their methods
are defined in other files, we have to load the methods of the
other files after the classes are recreated in this file
(recreation of classes deletes the old methods).
Use "::xo::library source_dependent" at the end of a file
when the classes are defined.