Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/vec_opaque.rs

@@ -0,0 +1,34 @@
+#[cxx::bridge]
+mod handle {
+    extern "C++" {
+        type Job;
+    }
+}
+
+#[cxx::bridge]
+mod ffi1 {
+    extern "C++" {
+        type Job;
+    }
+
+    extern "Rust" {
+        fn f() -> Vec<Job>;
+    }
+}
+
+#[cxx::bridge]
+mod ffi2 {
+    extern "C++" {
+        type Job = crate::handle::Job;
+    }
+
+    extern "Rust" {
+        fn f() -> Vec<Job>;
+    }
+}
+
+fn f() -> Vec<handle::Job> {
+    unimplemented!()
+}
+
+fn main() {}