pydantic_model_creator refactorization (#1745)

* naming is not working

* naming and description

* naming is not working

* naming and description

* include improvements from #1741

* remove print statements

* i should learn git better...

* type hints, recursion protector, naming, ....

should be ready for review

* unused import statements...

* test_early_init.py: naming of $defs changed

* move dataclasses to a dedicated place

* python 3.8 and 3.9: typing

* test computed fields, remove own PydanticMeta class

* remove print statements from tests

* re-add pydantic_model_creator docstring

it got lost during refactoring

* remove _stack from pydantic_model_creator

this is now handled by PydanticModelCreator

* make some methods private

* slim down dataclasses to only include necessary information

* remove unused imports, satisfy mypy

* add type annotation for ``from_pydantic_meta``

* better indexing of pydantic models

* remove dataclasses for field descriptions

pydantic_model_creator now accesses the fields directly

* remove forgotten line of #1465

* include optional in hashed value

* move dataclasses.py to descriptions.py

* stupid unused import.

* formatting