treesum: Simplify the path computations and make the visible output more consistent. Current format of digest lines: - No `=' if there was a severe error so that digest and/or size could not be computed. - Empty fields for digest and/or size of they were not computed on purpose: * for symbolic links * for special files

# Pure-Python-Build
[build-system]
requires = ["setuptools>=43.0", "wheel>=0.33"]
build-backend = "setuptools.build_meta"