Fixed handling of namespace packages in zip files

[jaraco]

• When constructing a MultiplexedPath, resolve submodule_search_locations to Traversable objects. Closes NotADirectoryError: MultiplexedPath only supports directories #287.

[jaraco]

Tests are failing on Windows, presumably because of backslashes as separators for zip subdirectories in submodules_search_paths.

[jaraco]

I confirmed that on Windows, submodules_search_locations uses backslashes:

>>> import pkg
>>> pkg.__spec__
ModuleSpec(name='pkg', loader=<_frozen_importlib_external.NamespaceLoader object at 0x0000021A5C97B3E0>, submodule_search_locations=_NamespacePath(['C:\\Users\\jaraco\\draft\\pkg.zip\\pkg']))

[jaraco]

Ignoring diffcov failure again, due to the problems with the library being loaded prior to testing.