[M8] reusable libraries: install layout + cargoxx-path deps

tests/e2e/pathDep/greeter/src/lib.cppm

@@ -0,0 +1,8 @@
+export module greeter;
+import std;
+
+export namespace greeter {
+auto hello(std::string_view who) -> std::string {
+    return std::format("Hello from greeter, {}!", who);
+}
+} // namespace greeter