native.repository_name() and ctx.workspace_name() are similar function but with different name

To get the workspace name of the call site, we do `native.repository_name()` in a macro and `ctx.workspace_name()` in a rule. The latter is behaving differently in the main repository probably (returning the name, whereas the repository name can be empty).

This is a [confusing API](https://stackoverflow.com/questions/47068989/how-to-compute-the-bazel-workspace-name-in-a-macro) and it needs to be cleaned.

Start by comparing native.repository_name() in macros with ctx.workspace_name() in rules, especially their behavior in the main repository. Determine the intended consistent API semantics, then update the relevant implementation and coverage so both entry points agree and the confusing naming or behavior is resolved.

Written by the indexing model from the issue text.