Fix Panic when Name can't be determined and check the filesystem to determine directory

[coreyja]

This fixes some bugs reported by @petobens, thanks so much for the reports!

The first is a panic when the last bit of the path as .., because it used Path::file_name and paniced on .., which is documented to return None.
https://doc.rust-lang.org/std/path/struct.Path.html#method.file_name
Now we return a None as well, and when we need to render the filename we use the input string instead so we should see .. un-modified in that case

The second fix is to better identify directories. Now instead of just checking for a trailing slash, we ask the file system as a fallback. This supports both local and absolute paths