Vendor dependencies

Let's see how I like this workflow.

vendor/thiserror/tests/test_lints.rs

@@ -0,0 +1,18 @@
+use thiserror::Error;
+
+pub use std::error::Error;
+
+#[test]
+fn test_unused_qualifications() {
+    #![deny(unused_qualifications)]
+
+    // Expansion of derive(Error) macro can't know whether something like
+    // std::error::Error is already imported in the caller's scope so it must
+    // suppress unused_qualifications.
+
+    #[derive(Debug, Error)]
+    #[error("...")]
+    pub struct MyError;
+
+    let _: MyError;
+}