In importlib.metadata, when a distribution file is corrupt and there is no
metadata file, calls to ``Distribution.metadata()`` (including implicit
calls from other properties like ``.name`` and ``.requires``) will now raise
a ``MetadataNotFound`` Exception. This allows callers to distinguish between
missing metadata and a degenerate (empty) metadata. Previously, if the file
was missing, an empty ``PackageMetadata`` would be returned and would be
indistinguishable from the presence of an empty file.
