cmdlib.py: move _GetQueryImplementation to end of file

_GetQueryImplementation() uses _QUERY_IMPL, which list all query type
implementations. By moving it to the end of the file, we ensure all such
classes are defined by the time we list them.

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>