Add support for C++20 modules

[bkryza]

This issue concerns only class diagrams, and the feature should enable at least:

• support for 'glob' pattern to filter translation units based also on modules
• enable filtering elements in the diagram based on whether they are exported or not
• enable drawing packages for modules in addition to packages for namespaces (maybe in some different style to differentiate)
• add test cases to make sure that clang-uml even works with modules at all

[bkryza]

Checkout these links:

• https://github.com/mathstuf/cxx-modules-sandbox
• https://en.cppreference.com/w/cpp/language/modules
• https://vector-of-bool.github.io/2019/03/10/modules-1.html
• https://www.kitware.com/import-cmake-c20-modules/