Files in this distribution.
:return: List of PackagePath for this distribution or None
Result is `None` if the metadata file that enumerates files
(i.e. RECORD for dist-info, or installed-files.txt or
SOURCES.txt for egg-info) is missing.
Result may be empty if the metadata exists but is empty.
Custom providers are recommended to provide a "RECORD" file (in
``read_text``) or override this property to allow for callers to be
able to resolve filenames provided by the package.