refactor *Source classes in tahoe_cp.py to have a basename() method rather than passing around (name, source) pairs

[daira]

Much of the code in src/allmydata/scripts/tahoe_cp.py​ passes around (name, source) pairs, where the name is the basename (i.e. last path component) of the source object. The source object should instead have a basename() method. (It is called that in Twisted's FilePath API.)

[warner]

The source object might be a pure filecap or dircap, and thus have no name. I think I wrote it this way (with tuples) as a reminder of that feature/property. I wouldn't object to moving this value into an property or method of the TahoeFileSource and LocalFileSource objects, but I'm not sure that basename() is the best name for it, if that's likely to imply that it always exists.

I think this assumption is the root cause for the #2329 regression.

[daira]

Well, the bad assumption was made for the existing tuple approach, in any case. I'm not insistent on calling it basename(); it could be called something like basename_if_present() or basename_or_None() if that helps.