Allow a callback function for the Source path parameter.

Allow for the path parameter in the migration YML to be call a function for additional logic to set a path prior to the migration.

src/Plugin/migrate/source/CSV.php

@@ -199,6 +199,17 @@ class CSV extends SourcePluginBase implements ConfigurableInterface {
    * @throws \League\Csv\Exception
    */
   public function initializeIterator() {
+    // Support a callback to return the path.
+    if (isset($this->configuration['path']['callback'])) {
+      if (is_callable($this->configuration['path']['callback'])) {
+        $this->configuration['path'] = call_user_func($this->configuration['path']['callback']);
+      }
+      else {
+        $message = 'The path callback function is not callable.';
+        throw new InvalidPluginDefinitionException($this->getPluginId(), $message);
+      }
+    }
+
     $header = $this->getReader()->getHeader();
     if ($this->configuration['fields']) {
       // If there is no header record, we need to flip description and name so