Don't include invalid paths in the depinfo for builtin backends

Author: bjorn3

compiler/rustc_interface/src/passes.rs

@@ -648,7 +648,11 @@ fn write_out_deps(
 
         if sess.binary_dep_depinfo() {
             if let Some(ref backend) = sess.opts.debugging_opts.codegen_backend {
-                files.push(backend.to_string());
+                if backend.contains('.') {
+                    // If the backend name contain a `.`, it is the path to an external dynamic
+                    // library. If not, it is not a path.
+                    files.push(backend.to_string());
+                }
             }
 
             boxed_resolver.borrow_mut().access(|resolver| {