"make install" does not install qpid_dispatch[_site.py] files in the correct location

Building and install qdr from source (cmake.... ; make install....) ends up putting qdr's python library modules in the wrong directory (not in the default python search path).

Specifically, these files are incorrectly installed in a "site-packages" sub-directory:

By default ubuntu's python does not use "site-packages" for installing 3rd party python packages - it uses "dist-packages" instead.

The default python paths in ubuntu are:

>>> import sys
>>> sys.path
['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', *'/usr/local/lib/python3.6/dist-packages'*, '/usr/lib/python3/dist-packages']

For example, proton correctly puts its python files in

/usr/local/lib/python3.6/dist-packages/

on ubuntu, while dispatch drops its packages into

/usr/local/lib/python3.6/site-packages/