Add support for redirecting for a namespace package.

[jaraco]

For discussion in #34, this PR proposes one way that the redirector might add support for namespace packages. This approach adds explicit support for manifesting a namespace package when the editable package is a submodule or package of said namespace package.

[pfmoore]

Please add documentation to the PR that explains how this would be used in practice. I'm not committing to accepting this idea at this point, but a prerequisite for considering it is understanding how it would look to the end user (i.e., the build backend writer), which means seeing the documentation.