PR# 14223 Improving override mechanism

When compiling a class which is in an override cluster, we might have the following scenarios:

1 - the class A overrides a class from a `library b', in that case all dependencies of `A' are resolved using the context of `library b'. In addition to the context of `library b', classes from the various override clusters of the project are also looked up on, but only classes that are not itself overriding another class (because if they were overriding an existing class, they should be properly resolved in the context of `library b').

2 - the class A does not override a class, dependencies are resolved using the overrides directive library context if present, otherwise the project context.

The above assume that a class cannot override more than one class, if 2 libraries provide the same class name, then the `overrides' directive on the `override' cluster have to be used to specify which library is overridden.