Python import scheme should be documented and/or enforced

Yes. It is mostly for people who import the code into their WORKSPACE. They might land it at `/third_party/bazel/rules_pkg` and want to normalize the paths so their Python code only has a single import path at the top.

I'm torn between a local guide to this or the overall "blaze ecosystem" style guide. I have an AI to take a crack at the latter, so I am inclined to put it there.

I am not sure how to auto-enforce it. I notice the problem when I try to import this into Google. Personally, I try very hard to avoid the `imports = ["."]` trick you mention because it tends not to scale. I can go into a full on rant about `configure` and how when you have multiple libraries that add to the include path and they all have a file named `config.h` you end up in trouble.

_Originally posted by @aiuto in https://github.com/bazelbuild/rules_pkg/pull/399#discussion_r685221893_