More pypfopt into src/pypfopt

[tschm]

Simplifies the application of generic tools a great deal... In src there shall be no init.py

[fkiraly]

I would prefer to not make too many changes for the time being - a change in folder may break workflows for people who have forked the package, for instance.

We also omit the src layer more often than not in the GC.OS managed packages, so the current state is more consistent.

[tschm]

This particular change has no effect as in pyproject we point to src/pypfopt rather than pypfopt. No import has to be changed

[fkiraly]

So if it has no effect, why change it?