chore: add submodule with relative URL to .gitmodules

[neersighted]

Successor to #4, which was not entirely correct and could not be amended as it was made off the master branch.

Introduce a relative URL submodule as a test fixture based on a branch on this same repository with no history.

Co-authored by @evanrittenhouse

[evanrittenhouse]

@neersighted I believe standalone_branch needs a git repo to count as a submodule, right? Currently it just has a README.

On a related note, is there a reason we don't want to just add another submodule to main similar to sample_namespace_packages?

This is in reference to python-poetry/poetry#6499.

[neersighted]

I'm not sure what you mean -- standalone_branch is a branch on this repo with disparate history and a README, indeed. However, it shares the same .git folder as this repo as it is just a branch on this repo referenced as a submodule.

Do you mean why we don't use a stand-alone repo as the test submodule? There's not a lot of point in doing that when we can just be self-contained and reference ourselves/I don't want to create another test fixture on the python-poetry org.