cleanup default locations

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?!

Start with src/extensions/score_metamodel/yaml_parser.py and inspect the defaults defined in docs() and the other locations mentioned in the issue. Trace how Bazel passes values to Python through environment variables, including file paths and JSON. Done means the default definitions and their configuration flow are consolidated into an agreed single approach.

Vom Indexierungsmodell aus dem Issue-Text verfasst.