Change directory structure for source_code to align with the PTB stand

[ArBridgeman]

Currently, the source code is in <root>/exasol_transformers_extension, but the convention for projects using the PTB is for it to be <root>/exasol/transformers_extension. In order to accommodate this unique structure, the source_code_path in the BaseConfig has been overridden in the noxconfig.py file.

Tasks

• Change the directory structure
• Update the noxconfig.py
• Check & alter any CI pipelines
• Check & alter the pyproject.toml
• Check & alter any documentation