Transfer path normalization breaks directory content transfers

[JoshKarpel]

Because pathlib.Path objects don't store trailing slashes, the mode of transfer_input_files where you put a trailing slash on a directory to transfer its contents can't be used with HTMap.

[JoshKarpel]

Given #216 , it should be possible to add a transfer_contents_only keyword argument to a TransferPath that captures this behavior. It's not clear to me whether the trailing-slash convention works with non-Condor file transfer. Need to investigate that.