This interface supports name completion, which is used primarily for
command line tools, etc. It provides a flat source of "names" in a
space. For example all of the classes in the classpath or all of the
variables in a namespace (or all of those).
NameSource is the lightest weight mechanism for sources which wish to
support name completion. In the future it might be better for NameSpace
to implement NameCompletion directly in a more native and efficient
fasion. However in general name competion is used for human interaction
and therefore does not require high performance.