Do not import non-namespaced classes and functions by default, since …

…importing a non-namespaced class in another non-namespaced class triggers a notice

src/PhpCsFixer/Rules.php

@@ -72,7 +72,7 @@ private static function getBaseRules(): array
             'explicit_string_variable'     => true,
             'fully_qualified_strict_types' => true,
             'function_to_constant'         => true,
-            'global_namespace_import'      => true,
+            'global_namespace_import'      => false,
             // 'header_comment'            => [ // Has too many issues atm
             //     'header' => '',
             // ],