Added support for C++20 module based packages in class diagrams (#101)

tests/t00071/src/lib1.cppm

@@ -0,0 +1,13 @@
+export module t00071.app.lib1;
+
+export namespace clanguml::t00071 {
+class B { };
+
+template <typename T> class BB {
+    T t;
+};
+
+namespace detail {
+enum class BBB { bbb1, bbb2 };
+} // namespace detail
+}