Rename crate::yubikey to crate::key

So that it doesn't conflict with the renamed `yubikey` crate.

src/builder.rs

@@ -9,9 +9,9 @@ use yubikey_piv::{
 
 use crate::{
     error::Error,
+    key::{self, Stub},
     p256::Recipient,
     util::{Metadata, POLICY_EXTENSION_OID},
-    yubikey::{self, Stub},
     BINARY_NAME, USABLE_SLOTS,
 };
 
@@ -90,7 +90,7 @@ impl IdentityBuilder {
         // No need to ask for users to enter their PIN if the PIN policy requires it,
         // because here we _always_ require them to enter their PIN in order to access the
         // protected management key (which is necessary in order to generate identities).
-        yubikey::manage(yubikey)?;
+        key::manage(yubikey)?;
 
         if let TouchPolicy::Never = touch_policy {
             // No need to touch YubiKey