Move the grolesync module to the main module for outsiders only.

The code looks solid @kristiaanvandeneynde, but as you've pointed out yourself. it does still needs tests. I'd especially like to see some kernel or web tests that demonstrate the intent of this change.

+++ b/tests/src/Kernel/GroupRoleSynchronizationTest.php
@@ -0,0 +1,93 @@
+  /**
+   * Tests whether a new global role syncs to group roles.
+   */
+  public function testGlobalRoleCreation() {
+    $group_roles = $this->entityTypeManager->getStorage('group_role')->loadMultiple();
+    $key = $this->getGroupRoleId('default', $this->userRole->id());
+    $this->assertArrayHasKey($key, $group_roles, 'Synchronized role found for default group type');
+    $key = $this->getGroupRoleId('other', $this->userRole->id());
+    $this->assertArrayHasKey($key, $group_roles, 'Synchronized role found for other group type');
+    $this->assertEquals($this->userRole->label(), $group_roles[$key]->label(), 'Synchronized roles share the same label');
+  }

Method comment and name are contradicting each other.

testGlobalRoleCreation !== Tests whether a new global role syncs to group roles

Besides that, both mention the creation of a role, but I'm not seeing any roles getting created.

Side note:
Tests should consist of 3 specific blocks of code:
- Precondition (e.g. Optional creation of a group to test against)
- Execution (e.g. creation of the roles)
- Postcondtion (e.g. assertions that prove the roles have been created and synchronised)

+++ b/tests/src/Kernel/GroupRoleSynchronizationTest.php
@@ -43,25 +29,49 @@ class GroupRoleSynchronizationTest extends GroupKernelTestBase {
+    $role = $this->createUserRole('editor');
+    $role->set('label', 'Super-editor');
+    $role->save();

Nitpick: This could use a blank line between the creation of the role (pre condition) and the editing of the role (execution) to separate those two concepts a little clearer.

Looks good otherwise.