Don't append multiple duplicate reverse deps for package name

reverse_deps_map stores duplicate reverse deps, certainly not a real issue, but it could lead to some performance / memory issues:

[src/lib.rs:90] reverse_deps_map.clone() = {
    "python2-ndg-httpsclient": [
        "python-urllib3",
        "python-urllib3",
        "python-urllib3",
    ],
}