Bug#1096186: fails with PYTHONSAFEPATH

[VA]

Package: game-data-packager-runtime
Version: 84

When running a game with PYTHONSAFEPATH=y, this error occurs at start:

```
Traceback (most recent call last):
   File "/usr/games/serious-sam-tfe", line 25, in <module>
     from gdp_launcher_base import (
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     ...<3 lines>...
     )    # noqa
     ^
ModuleNotFoundError: No module named 'gdp_launcher_base'
```

The solution would be to insert some code like 
`sys.path.append(os.path.dirname(__file__))`, or better, properly 
package the source files into something like debian's dist-packages folder.