Use repo-relative labels instead of @rules_python-qualified labels

[rickeylev]

Currently, the code base uses a mixture of @rules_python//foo:bar and //foo:bar labels. Having a mixture is innocuous, but introduces an unnecessary inconsistency in style.

Let's just normalize on using the // labels within our project instead of @rules_python//. It's shorter and makes it clear the references are intra-repo instead of inter-repo.

[alexeagle]

There is a case where the fully-qualified labels are needed though - when they are resolved from String -> Label in the user's context. It requires a bit of thought and care (or sufficient automated tests to catch this, which I think we probably have)

[rickeylev]

Good point. Let's make a habit of dropping a comment where we have to use the repo-qualified name for technical reasons.