Conform to OPA 0.61.0. (#118)

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>

tests/parser/cases/import/import.yaml

@@ -224,13 +224,13 @@ cases:
     rego: |
       package test
       import foo
-    error: "import path must begin with one of: {data, future, input}"
+    error: "import path must begin with one of: {data, future, input, rego}"
 
   - note: invalid-beginning-1
     rego: |
       package test
       import foo.bar
-    error: "import path must begin with one of: {data, future, input}"
+    error: "import path must begin with one of: {data, future, input, rego}"
 
   - note: missing-field-1
     rego: |