linting: Check format_source argument is a defined input file

e.g. This would have caught this error of mine:

https://github.com/peterjc/pico_galaxy/commit/734d298ac56af21380e92b3985ca971fc73e577b

I was replacing `format="input"` given the planemo warning:

```
.. WARNING: Using format='input' on output data, format_source attribute is less ambiguous and should be used instead.
```