Fix #2893: Issues with extension's Python namespace packages

The `ckanext` Python package is installed by CKAN as a namespace
package. However, the templates for creating an extension do not
declare that package as a namespace package. Extensions created
from the templates therefore break the virtualenv.

The fix is to make sure that `ckanext` is declared as a namespace
packages in the extension's `setup.py`.

ckan/pastertemplates/template/setup.py_tmpl

@@ -54,6 +54,7 @@ setup(
     # You can just specify the packages manually here if your project is
     # simple. Or you can use find_packages().
     packages=find_packages(exclude=['contrib', 'docs', 'tests*']),
+    namespace_packages=['ckanext'],
 
     # List run-time dependencies here.  These will be installed by pip when your
     # project is installed. For an analysis of "install_requires" vs pip's