Fix invalid Python import for qualified package name

Created by: loleary

Fix #13648 (closed)

Change from packagePath() to packageName when generating the Python import statement.

@spacether

Test command:

$ java -jar ./modules/openapi-generator-cli/target/openapi-generator-cli.jar \
    generate \
        -i ./modules/openapi-generator/src/test/resources/3_0/petstore.yaml \
        -g python \
        --additional-properties packageName='my.openapi' \
        -o /tmp/issue-13648

Before:

$ tail -n2 /tmp/issue-13648/my/openapi/model/pet.py
from my/openapi.model.category import Category
from my/openapi.model.tag import Tag

After:

$ tail -n2 /tmp/issue-13648/my/openapi/model/pet.py
from my.openapi.model.category import Category
from my.openapi.model.tag import Tag

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