reference patch locally instead of relying on drupal.org

.gitignore

 # ignores all .lock recursive from the base folder
 **.lock
-modules
-themes
+/modules
+/themes

composer.json

@@ -94,7 +94,7 @@
         },
         "patches": {
             "drupal/externalauth": {
-              "ISTWCMS-2267: Remove provider prefix to username (saml_)": "https://www.drupal.org/files/issues/2798323-externalauth-11.patch" 
+              "ISTWCMS-2267: Remove provider prefix to username (saml_)": "patches/modules/externalauth/2798323-externalauth-11.patch" 
             }
         }
     }

patches/modules/externalauth/2798323-externalauth-11.patch

+diff --git a/src/ExternalAuth.php b/src/ExternalAuth.php
+index 2811d48..b49e1b3 100644
+--- a/src/ExternalAuth.php
++++ b/src/ExternalAuth.php
+@@ -91,7 +91,8 @@ public function login($authname, $provider) {
+    * {@inheritdoc}
+    */
+   public function register($authname, $provider, $account_data = array(), $authmap_data = NULL) {
+-    $username = $provider . '_' . $authname;
++    $username = $authname;
++
+     $authmap_event = $this->eventDispatcher->dispatch(ExternalAuthEvents::AUTHMAP_ALTER, new ExternalAuthAuthmapAlterEvent($provider, $authname, $username, $authmap_data));
+     $entity_storage = $this->entityManager->getStorage('user');
+ 
+@@ -104,7 +105,6 @@ public function register($authname, $provider, $account_data = array(), $authmap
+     $account_data = array_merge(
+       [
+         'name' => $authmap_event->getUsername(),
+-        'init' => $provider . '_' . $authmap_event->getAuthname(),
+         'status' => 1,
+         'access' => (int) $_SERVER['REQUEST_TIME'],
+       ],
+@@ -158,7 +158,7 @@ public function linkExistingAccount($authname, $provider, UserInterface $account
+     // If a mapping (for the same provider) to this account already exists, we
+     // silently skip saving this auth mapping.
+     if (!$this->authmap->get($account->id(), $provider)) {
+-      $username = $provider . '_' . $authname;
++      $username = $authname;
+       $authmap_event = $this->eventDispatcher->dispatch(ExternalAuthEvents::AUTHMAP_ALTER, new ExternalAuthAuthmapAlterEvent($provider, $authname, $username, NULL));
+       $this->authmap->save($account, $provider, $authmap_event->getAuthname(), $authmap_event->getData());
+     }