feat: Raise error when problematic use of the if keyword is encountered

[jalseth]

Using the if keyword without also using the contains keyword makes the rule name in the OPA AST an emptry string, which causes conftest to inadvertently skip over tests leading to inaccurate results.

open-policy-agent/opa#6509

[anderseknert]

I guess this works, but since you have parsed the modules already, you could also iterate over the rules and check each rule head either for the presence of a name, or whether the ref contained has more than one element.

The AST representation of a policy like you use for testing:

package main

import future.keywords.if

deny[msg] if {
    msg := "foo"
}

would look something like this for the rule head:

"head": {
        "value": {
          "type": "boolean",
          "value": true
        },
        "ref": [
          {
            "type": "var",
            "value": "deny"
          },
          {
            "type": "var",
            "value": "msg"
          }
        ]
}

[jalseth]

@anderseknert thanks for the insight to the AST, it made it easy to set up without regexp.

[boranx]

lgtm!