Ensure no unresolved types in the definition files (#6521)

* Ensure no unresolved types in the definition files

* Address feedback and general cleanup

* Move import from @constants/x to @constants where relevant

* Remove unneeded "import as"

types/utils/index.d.ts

@@ -1,6 +0,0 @@
-// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
-// See LICENSE.txt for license information.
-
-type Dictionary<T> = {
-    [key: string]: T;
-};