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.

Start in mypy/config_parser.py around the type callback at line 75 and trace how the detected project config root is passed into option handling. Verify that relative mypy_path values and cache_dir defaults use the project config directory, while relative paths in global user config files use that file's directory without making it the root for default settings.