fix: preserve relationships in APT package index

[wmdhs12138]

Summary

• preserve Debian relationship fields when generating Packages.gz
• include Pre-Depends, Depends, Recommends, Suggests, Enhances, Conflicts, Breaks, Replaces, and Provides when present
• omit absent relationship fields instead of emitting an empty Depends

Problem

read_control() extracts these fields from each .deb, but the manually assembled index stanza previously discarded everything except Depends. For example, the native opencode package contains:

Conflicts: opencode-glibc, opencode-compressed

The generated Packages.gz omitted it. With the flat APT source enabled, this metadata mismatch can make APT select the repository copy of an already-installed same-version package again.

Verification

• python -m py_compile tools/gen-packages-index.py
• generated an index from opencode_1.18.29_aarch64.deb
• confirmed the generated stanza retains Conflicts: opencode-glibc, opencode-compressed
• git diff --check