Interpret paths in known-model.json relative to the JSON file

[Wuestengecko]

The current behavior of interpreting relative paths in a known-model.json file relative to CWD is very unintuitive, and can easily lead to confusion if CWD doesn't match expectations. capellambse.repl tries to work around that by switching to the project root, so that e.g. python -m capellambse.repl test-6.0 works as expected; however, that introduces the new problem that cd /some/model; python -m capellambse.repl . doesn't work anymore.

We should adjust the loader behavior so that it interprets paths relative to the location of the JSON file that they came from. If the path didn't come from a JSON file, we should still fall back to using CWD, to still allow e.g. a stringified JSON object to be passed in as command line argument.