Item reference is invalid - when relying on target namespace, or when using explicit namespace

```

```

and

```

```

both fail with a "Item reference is invalid: not found" from MessageValidator

patch written locally against 8.0.0, will create pull request after it is validated in flowable build

Start by reproducing both itemDefinition and message examples in the Flowable build, then inspect MessageValidator and its item-reference lookup. Done means both target-namespace and explicit-namespace forms validate without the "Item reference is invalid: not found" error.

Written by the indexing model from the issue text.