Mechanism for renaming/aliasing classes

Mechanism for renaming/aliasing classes

Sometimes it is desirable to change a class name that is still referenced
in extensions or config files (e.g. for consistency, as in I507ba00a).
PHP's class_alias() function can help in preserving backward compatibility;
however, creating an alias first requires that the class be loaded.

Hence this is implemented in AutoLoader. Lazy loading continues to work,
the list of class names is still maintained in a central location, and
optionally, deprecation warnings can be generated.

Change-Id: I1d3fb04a448647b5be10bed7fec05238b9fc6fc7