(PUP-659) Remove ability to have multiple names in a scope

This removes the ability for a scope to have multiple namespaces.
The mechanism was not used for anything productive and resulted
in performance overhead.

This also removes serching through multiple namespaces (given
by a scope, as well as all wider namespaces of a given name). This
was also not used because references are now always exactly to
the referenced element (except for variables which are handeld a
different way).

There were many tests that tested the implementation of the
TypeCollection class and its support for features that were not used.

There will be additional cleanup in this area, such as changing the
API.

This commit makes it just as fast to reference with names that are not
using an initial :: as it is without. Before this change references
without :: took 1.5x as long.