cleanup default locations

[AlexanderLanin]

example:

• metamodel.yml default is configured in src/extensions/score_metamodel/yaml_parser.py
• other defaults are defined in docs()
• others in 10 other locations :D

related:

• we pass data from bazel to python via environment variables containing values, environment variables pointing to files, environment variables containing json. Maybe that can be simplified?

We should have a single place for defaults?!