Given a path to a file in this distribution, return a SimplePath
to it.
This method is used by callers of ``Distribution.files()`` to
locate files within the distribution. If it's possible for a
Distribution to represent files in the distribution as
``SimplePath`` objects, it should implement this method
to resolve such objects.
Some Distribution providers may elect not to resolve SimplePath
objects within the distribution by raising a
NotImplementedError, but consumers of such a Distribution would
be unable to invoke ``Distribution.files()``.