Refactoring: centralize uri computation in solidlsp

[MischaPanch]

The code computing

        absolute_file_path = Path(self.repository_root_path, relative_file_path)
        uri = absolute_file_path.as_uri()

is repeated all over the place in slight variations. We also may compute absolute paths in various places. We should introduce an internal _FilePath dataclass with entries relative_path, absolute_path, uri and a method _get_file_path(relative_path), and use that everywhere.

Affects code in ls.py and downstream in solidlsp