Added support for class diagram filtering based on C++20 modules (#195)

tests/t00070/src/common.cppm

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+export module t00070;
+export import t00070.lib1;
+export import t00070.lib2;
+
+export namespace clanguml::t00070 {
+class A {
+    int get() { return a; }
+
+    int a;
+};
+}

tests/t00070/src/lib1.cppm

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+export module t00070.lib1;
+
+export namespace clanguml::t00070 {
+class B { };
+
+template <typename T> class BB {
+    T t;
+};
+
+enum class BBB { bbb1, bbb2 };
+}

tests/t00070/src/lib2.cppm

@@ -0,0 +1,11 @@
+export module t00070.lib2;
+
+export namespace clanguml::t00070 {
+class C { };
+
+template <typename T> class CC {
+    T t;
+};
+
+enum class CCC { ccc1, ccc2 };
+}