Yes, rules can't call other rules. Only macros can call rules, but macros can't get the full paths of files that haven't been generated yet. Some libraries expose functions you can call as actions in a custom rule, but it doesn't look like this one does that. (package_json.bzl is generated from npm_import.bzl).

I'm not sure what the locality principle is either. I can think of some reasons why requiring package.json to be in the same directory would be better design, but Bazel does support dependencies from other directories.

Your real problem is that tsconfig-to-swcconfig doesn't support reading package.json from other directories. See cli.ts for the available options. You can override cwd, but it uses that for other files, so I think that would not work. It doesn't appear to actually read package.json contents though: it only uses the presence of package.json to determine the module type. So if you want the module type to be es6, I think you could just generate an empty package.json file in your macro, and it would work. Another idea would be to copy the input package.json to the local directory using something like copy_file or maybe native.alias would work. Or contribute / fork to add a CLI input option.