Vendor dependencies

Let's see how I like this workflow.

vendor/thiserror/tests/ui/from-not-source.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub struct Error {
+    #[source]
+    source: std::io::Error,
+    #[from]
+    other: anyhow::Error,
+}
+
+fn main() {}