Refactor SignerSource to expose DerivationPath to other kinds of signers (#16933)

* One use statement

* Add stdin uri scheme

* Convert parse_signer_source to return Result

* A-Z deps

* Convert Usb data to Locator

* Pull DerivationPath out of Locator

* Wrap SignerSource to share derivation_path

* Review comments

* Check Filepath existence, readability in parse_signer_source

clap-utils/src/nonce.rs

@@ -1,5 +1,7 @@
-use crate::{input_validators::*, offline::BLOCKHASH_ARG, ArgConstant};
-use clap::{App, Arg};
+use {
+    crate::{input_validators::*, offline::BLOCKHASH_ARG, ArgConstant},
+    clap::{App, Arg},
+};
 
 pub const NONCE_ARG: ArgConstant<'static> = ArgConstant {
     name: "nonce",