Treat relative paths in config file relative to config file, not cwd

When using mypy_path = src it should be treated relative to the config file (if any), not the current working directory.
This also applies to the default of cache_dir (.cache_dir), which currently gets created in the current working directory always.

I.e. a detected/found (project) config file could be treated as the root for options with relative defaults also.

As for mypy_path this could be achieved in the type callback, which then however would need the config file / root as a new argument:
https://github.com/python/mypy/blob/84126ab9985844d8cda3809688034af895fdaf7c/mypy/config_parser.py#L75

As for global user config files, it should treat relative files therein relative to it, but it should not be used as a root for default settings like cache_dir.

Beginne in mypy/config_parser.py beim type callback in Zeile 75 und verfolge, wie das erkannte Root-Verzeichnis der Projektkonfiguration an die Optionsverarbeitung übergeben wird. Überprüfe, dass relative mypy_path-Werte und cache_dir-Standardwerte das Verzeichnis der Projektkonfiguration verwenden, während relative Pfade in globalen Benutzerkonfigurationsdateien das Verzeichnis dieser Datei verwenden, ohne es zum Root-Verzeichnis für Standardeinstellungen zu machen.