ast: parse import without package in resolveRefs

[onelittlenightmusic]

Before, resolveRefs ignored Imports when Package is nil.
This caused opa eval --import option to be ignored without --package option.

This PR makes query compiler generate temporary package with name of ""
(empty string) when package is nil but import one or many imports are provided.

For example, we have two files.

filter1.rego

package f

getFamily(i) = { rtn |
  rtn := i.characters[_].family
}

getChildren(family) = { rtn |
  rtn := family[_].children
}

data.json

{
  "characters": [
    {
      "id": 0,
      "name": "Marty McFly",
      "cast": "Michael J. Fox",
      "family": {
        "children": [
          "Marty McFly Jr.",
          "Marlene McFly"
        ]
      },
      "occupation": "student"
    }
  ]
}

Before, we had this error because of no --package option.

opa eval -I --format=pretty -d filter1.rego --import 'data.f' 'f.getChildren(f.getFamily(input))' < data.json
2 errors occurred:
1:15: rego_type_error: undefined function f.getFamily
1:1: rego_type_error: undefined function f.getChildren

After this PR, we can use --import option without any explicit --package option.

opa eval -I --format=pretty -d filter1.rego --import 'data.f' 'f.getChildren(f.getFamily(input))' < data.json
[
  [
    "Marty McFly Jr.",
    "Marlene McFly"
  ]
]

Fixes #3228

Signed-off-by: Hiro Osaki hiroyuki.osaki@gmail.com

Thank you for reviewing this PR.

[tsandall]

LGTM. Thank you @onelittlenightmusic 🙏