Fix invalid Python import for qualified package name

[loleary]

Fix #13648

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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[spacether]

Looks great, thank you for the fix and the verification test!