conditional downstream build

the dependency between repositories A and B might be isolated in just one module (or a couple of modules) in the repository A. There is no need to run the downstream build of the repository B unless the PR touches the "interesting" module(s) in the repository A.

Start by tracing how the GitHub Action identifies dependencies between repositories A and B and where it triggers the downstream build. Determine how changed modules in repository A could be identified and how the relevant workflow entry points are tested. Done means downstream builds run only when the specified interesting modules are touched, with behavior documented by tests.

Written by the indexing model from the issue text.