Implement `import rego.v1`
https://www.openpolicyagent.org/docs/latest/policy-language/#the-regov1-import
- `if` required before rule body
- import rego.v1 automatically imports future.keywords
- handle import shadowing
- data, input cannot be shadowed
- deprecated functions as disallowed
- rules must have assignment or body
- `contains` required for parital set
Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>
