feat: phase2 split-keys: write info file

Write a file for each parameter which contains the path to the input
file. This can be used to verify that the files were split correctly.

filecoin-proofs/src/bin/phase2.rs

@@ -1564,6 +1564,16 @@ fn main() {
                         |_| panic!("failed to write meta information to {}", meta_path),
                     );
                 }
+
+                // The info file contains the filename the parameter was created of.
+                {
+                    let info_path = format!("v{}-{}.info", parameter_cache::VERSION, &identifier);
+                    println!("writing info to file: {}", info_path);
+                    let mut info_file = File::create(&info_path)
+                        .unwrap_or_else(|_| panic!("failed to create {}", info_path));
+                    writeln!(&mut info_file, "{}", input_path)
+                        .unwrap_or_else(|_| panic!("failed to write info data to {}", info_path));
+                }
             }
             _ => unreachable!(),
         }