missing search for sphinxcontrib.plantuml python module

the below diff will fix this, BUT we must also add support for namespaced modules in the find_python_module function

with current code it will report the path as /usr/lib/python3.6/site-packages/sphinxcontrib, this is correct, but then import plantuml must be used, which is wrong. it must resolve to /usr/lib/python3.6/site-packages.

aka: foo.bar.module, we strip /foo/bar from the path

diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 965f2cf..6966e94 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -34,6 +34,7 @@ else()
        include(find_python_module)
        find_python_module(breathe)
        find_python_module(sphinx)
+       find_python_module(sphinxcontrib.plantuml)
 
        # do not change the ENV, doesn't work with add_custom_target
        # https://cmake.org/Bug/view.php?id=5145