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.

mypy/config_parser.py の 75 行目付近にある type callback から始め、検出されたプロジェクト設定のルートがオプション処理にどのように渡されるかを追跡します。相対 mypy_path 値と cache_dir のデフォルト値がプロジェクト設定ディレクトリを使用することを確認し、一方でグローバルユーザー設定ファイル内の相対パスは、そのファイルのディレクトリを使用しつつ、それをデフォルト設定のルートにはしないことを確認します。