Add unconverted_reasons_from_heuristics fields to the Module message

A heuristics is implemented in the build/bazel/scripts/bp2build_progress/bp2build_progress.py script to determine the reason for why a module is not converted.

Test: b build //build/bazel/scripts/bp2build_progress:bp2build_progress

Change-Id: I9d4eaf309dbf26bbb6de18e1af0d9cdc8fe09e94