Preinstalled packages

`libpcap` is blacklisted on Mac/Homebrew:

```
$ brew info libpcap
Apple distributes libpcap with OS X, you can find it in /usr/lib.
```

We can currently override the name of package, in case the native package has a different name from the Buzzy package name (for instance, [pkg-config on RHEL](https://github.com/redjack/buzzy-core/blob/master/.buzzy/repo.yaml#L4)).  We also need a way to say "hey man this package is already present, don't try to install it!"