Re-use joinpath logic in Traversable and MultiplexedPath

[jaraco]

Inspired by #248 (comment)

• Inline the flattening of descendants.
• Move ResourceContainer.joinpath to Traversable.joinpath, providing a concrete implementation.
• Refactor to converge descendants to a single type and rely on Path.parts for getting the parts.
• Replace StopIteration with a TraversalError for use in capturing a failed traversal.
• In readers.MultiplexedPath, re-use Traversable.joinpath.

[FFY00]

I had the look at the PR and it seems good. Thank you for splitting each logical change into its own commit.

I only have one note.

[FFY00]

The only way for this to happen is no arguments being passed, no? Wouldn't checking for that directly make the code easier to read?

[jaraco]

I don't think that's correct. A StopIteration would occur if none of the traversable.name == target in the matches above (including if self.iterdir() yields no results). It's trapping StopIteration on next(matches), not *names. *names can be empty. Maybe only the exception should be trapped for next(matches).

[FFY00]

Ah, right, that makes sense.