Documentation generation for Builtin Symbols with the same name and different contexts.

[mmatera]

This PR improves the way in which we walk the mathics.builtin package to build the documentation. With these changes

• some tests that are loaded twice in master are now loaded just once (this reduces in ~300 the number of doctests reported when make doctest runs ).
• fix the issue in the documentation generation that prevents symbols with the same name and different contexts be loaded.
• Makes the algorithm more deterministic, simpler, and allows to decompose MathicsMainDocumentation.doc_parts in simpler functions. (as proposed in #984)

[mmatera]

Already included in previous merged PR