Wiz freeze option lock down requirements instead of environment mapping

[buddly27]

Right now, running wiz freeze foo bar -o /tmp will create a definition with a combination of all environment variable resulting from the context resolution:

{
    "identifier": "foo-bar-fusion",
    "description": "This package combines foo and bar",
    "environ": {
        # Lots of environment variables
    }
}

It would be great to have an option to simply freeze the package requirements instead:

{
    "identifier": "foo-bar-fusion",
    "description": "This package combines foo and bar",
    "requirements": [
        "foo==2.0.3",
        "bar==1.2.3"
    ]
}