Currently, local module discovery is broken with luigi installed from
this derivation.
In the documentation, you can see the following command line syntax
used:
```
luigi --module module_name ...
```
However, currently this will result in an error:
```
ModuleNotFoundError: No module named 'module_name'
```
However, if the call is prepended with this:
```
PYTHONPATH=.:$PYTHONPATH
```
then it will work as expected.
This patch makes this the default behaviour.
a84a457f62