[Routing] Moving include instruction to separate method

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #12106
| License       | MIT
importing another file is done with ``import``, not with ``load`` (otherwise you cannot import a file in a different format)