[BUG] [Python] Misplaced __init__.py in client code when period is specified in package name

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

a) Generating a python client with both --model-package and --api-package parameters containing a '.' creates the api and models init.py file in the wrong directory.
Options --model-package pkg.models and --api-package pkg.api generate the directories openapi_client/pkg.models and openapi_client/pkg.api, in addition to openapi_client/pkg/models and openapi_client/pkg/api.

b) The init.py for the api contains wrong import lines, while the "model" one is correct.

--- openapi_client/pkg.models/init.py:

import models into model package

from openapi_client.pkg.models.error import Error
from openapi_client.pkg.models.pet import Pet

--- openapi_client/pkg.api/init.py ("openapi_client." missing form package name):

import apis into api package

from pkg.api.pets_api import PetsApi

openapi-generator version

Confirmed with v4.0.0-beta3

OpenAPI declaration file content or url

Using standard petstore.json

Command line used for generation

openapi-generator generate -g python -i petstore.json -o output --model-package pkg.models --api-package pkg.api

Steps to reproduce

Run the command above on the standard petstore.json spec file

Related issues/PRs

a) PR #2508 fixes a similar issue in the server code.

Suggest a fix

Run the provided openapi-generator command with petstore.json, --model-package pkg.models, and --api-package pkg.api, then inspect the generated openapi_client/pkg.models and openapi_client/pkg.api paths. Compare their init.py locations and import lines with the expected package structure, and review PR #2508 for the related server-side fix; done means no extra dotted-directory packages and correct imports.