fix(fastapi): use correct impl & package vars on custom user values

[hegerdes]

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Changes

This fixes an error when the user supplys custom values for fastapiImplementationPackage and packageName in the python-fastapi generator.

Currently when the user sets these, the implPackage will be generated on the wrong path (using the default package name) and starting the uvicorn python server would fail, due to wrong imports.

The behavior can be tested with these commands run from the projects root dir:

# Latest upstream
docker run --rm -it --user $(id -u $USER):$(id -g $USER) \
  -v ${PWD}:/local openapitools/openapi-generator-cli generate \
  -i /local/modules/openapi-generator/src/test/resources/3_1/nodesc.yaml \
  -g python-fastapi \
  -o /local/out/ \
  --additional-properties=packageName=my_name,fastapiImplementationPackage=logic

# Fixed version
docker run --rm -it --user $(id -u $USER):$(id -g $USER) \
  -v ${PWD}:/local hegerdes/debug:openapi-generator generate \
  -i /local/modules/openapi-generator/src/test/resources/3_1/nodesc.yaml \
  -g python-fastapi \
  -o /local/out/ \
  --additional-properties=packageName=my_name,fastapiImplementationPackage=logic

Please compare the src/<moduleName> and src/<moduleName>/impl names.

NOTE: There are existing tests, unfortunately these contained the same error as in the source-code. The source and test have been adapted and the handling of implPackage now aligns more to the handling of other attributes like packageName.

cc: @cbornet @tomplus @krjakbrjak @fa0311 @multani @krjakbrjak