opa inspect fails on unknown Rego functions

[johanfylling]

If the inspected bundle contains a module with a reference to an unknown function (expected to be present through other means at eval-time), inspection will fail.

E.g. the following module:

package play
import data.x.y

p {
    y(1) == true
}

will fail with:

policy.rego:5: rego_unsafe_var_error: expression is unsafe