Rework project structure: form a proper Python package

[atemate]

TL;DR
Let's modify template so that relative imports work:

<project-slug>
│   └── main.py
│    │    └── sub_package
│    │    │     └── train.py

from .main import *
from .sub_package.train import *

--
Problem:
We have over-complicated rules of naming (only dashes allowed for slug, only underscores allowed for code dir. At the same time, we don't allow users to address Python code as from a local package. Instead, we use hacks as -e PYTHONPATH=<project-dir> or sys.path.append('<project-dir>').

• Pros of using the hacks: code directory name can be fixed to src to simplify the template considerably
• Cons of using the hacks: to develop locally, i.e. to set up your local IDE, you'd need to use the same hacks, different across IDEs. Plus, hard to understand as it's not a stnadard way.

In turn, we should use a standard way of forming a Python package via setup.py. Note, in this case code dir still can be named with dashes.