Buildifier should sort package_group packages ignoring leading "-" characters

[netdpb]

In a package_group, the packages list contains positive and negative package patterns. With # keep sorted, negative ones all sort below positive ones:

    packages = [
        "//java/aaa/bbb/...",
        "//java/ddd/eee/...",
        "//java/ggg/hhh/...",
        "-//java/aaa/bbb/ccc",
        "-//java/ddd/eee/fff",
    ]

This is especially a problem for large package list arguments.

Instead, it would be better to sort them together:

    packages = [
        "//java/aaa/bbb/...",
        "-//java/aaa/bbb/ccc",
        "//java/ddd/eee/...",
        "-//java/ddd/eee/fff",
        "//java/ggg/hhh/...",
    ]