Add proper mirror support (#74)

* Add rewrite support and fix existing tests

* Add unit tests for policy matching

* Compile regexes up front and store policies in map

* Add test for verify flow with mirror

* Rename ImageName -> ResolvedName

And only set it when necessary

* Rename Rewrite -> Replacement

but keep it as rewrite in the yaml

pkg/policy/testdata/mappings/rewrite-loop/mapping.yaml

@@ -0,0 +1,14 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^yin/(.*)$"
+    rewrite: "yang/$1"
+  - pattern: "^yang/(.*)$"
+    rewrite: "yin/$1"