Update parser to support rules of the form <var> = <term containing ref>

[tsandall]

Rules can be written in the form <var> = <ground term>. This is useful for defining constants; it would be nice if we could do the same with terms containing refs. Currently, the parser will return an error because it deems the term as being non-ground (since references have not been resolved yet).

One solution would be to update the parser to allow non-ground terms. If the user did include a variable in the value and that variable was not eventually resolved, the compiler should catch the error when performing the safety checks on the head.

Example parse error:

torin:~$ opa run test.rego
test.rego:7: expected rule (annotations_versioned = {
    "metadata": {
        "annotations": annotations,
        "resourceVersion": object.metadata.resourceVersion
    }
} must be declared inside a rule)

Example input:

package io.k8s

import object

annotations = {}

annotations_versioned = {
    "metadata": {
        "annotations": annotations,
        "resourceVersion": object.metadata.resourceVersion
    }
}