[Draft] Improve C++20 modules support

tests/projects/c++/modules/stl_headerunit/src/hello.mpp

@@ -0,0 +1,11 @@
+module;
+
+export module hello;
+
+import <iostream>;
+
+export namespace hello {
+    void say(const char* str) {
+        std::cout << str << std::endl;
+    }
+}

tests/projects/c++/modules/stl_headerunit/src/main.cpp

@@ -0,0 +1,6 @@
+import hello;
+
+int main() {
+    hello::say("hello module!");
+    return 0;
+}

tests/projects/c++/modules/stl_headerunit/xmake.lua

@@ -0,0 +1,4 @@
+set_languages("c++20")
+target("stl_headerunit")
+    set_kind("binary")
+    add_files("src/*.cpp", "src/*.mpp")

tests/projects/c++/modules/user_headerunit/src/header.hpp

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace hello {
+    extern constexpr auto FOO = "Hello";
+}

tests/projects/c++/modules/user_headerunit/src/hello.mpp

@@ -0,0 +1,10 @@
+module;
+#include <cstdio>
+
+export module hello;
+
+export namespace hello {
+    void say(const char *arg) {
+        printf("%s\n", arg);
+    }
+}

tests/projects/c++/modules/user_headerunit/src/main.cpp

@@ -0,0 +1,7 @@
+import hello;
+import "header.hpp";
+
+int main() {
+    hello::say(hello::FOO);
+    return 0;
+}

tests/projects/c++/modules/user_headerunit/xmake.lua

@@ -0,0 +1,5 @@
+set_languages("c++20")
+target("user_headerunit")
+    set_kind("binary")
+    add_headerfiles("src/*.hpp")
+    add_files("src/*.cpp", "src/*.mpp")