Buildifier should sort package_group packages ignoring leading "-" characters

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/...",
]
```