Fix error during expansion of !!merge <<: anchor tags (#4383)

* WIP

* Fix merge corner cases

* Add test for explicit !!merge tag

* Fix tests

* Cleanup

* Cleanup

* Fix deanchoring lists

* Add test case for keeping comments

* Add MapEntrySetter and fix json marshalling after deanchoring

* Keep duplicated keys

* Move MergeTag definition to yaml alias

* Remove go-spew from api

* Add support for sequence nodes on merge tags

* Add docstring to MapEntrySetter.Key

* Add docstring to MapEntrySetter struct

* Add tests to MapEntrySetter

* Fix duplicate merge key

* Revert whitespace changes on forked go-yaml

* Remove AssocMapEntry function

* Refactoring merge order

* Return errors on VisitFields and PipeE

* Add tests for each non-conforming map merges

kyaml/kio/byteio_reader_test.go

@@ -743,7 +743,7 @@ items:
 `},
 			},
 		},
-		"listWithAnchors": {
+		"mergeAnchors": {
 			input: []byte(`
 apiVersion: v1
 kind: DeploymentList
@@ -764,7 +764,59 @@ items:
   metadata:
     name: deployment-b
   spec:
-    *hostAliases
+    <<: *hostAliases
+`),
+			exp: expected{
+				sOut: []string{`
+apiVersion: v1
+kind: DeploymentList
+items:
+- apiVersion: apps/v1
+  kind: Deployment
+  metadata:
+    name: deployment-a
+  spec:
+    template:
+      spec:
+        hostAliases:
+        - hostnames:
+          - a.example.com
+          ip: 8.8.8.8
+- apiVersion: apps/v1
+  kind: Deployment
+  metadata:
+    name: deployment-b
+  spec:
+    template:
+      spec:
+        hostAliases:
+        - hostnames:
+          - a.example.com
+          ip: 8.8.8.8
+`},
+			},
+		},
+		"listWithAnchors": {
+			input: []byte(`
+apiVersion: v1
+kind: DeploymentList
+items:
+- apiVersion: apps/v1
+  kind: Deployment
+  metadata:
+    name: deployment-a
+  spec: &hostAliases
+    template:
+      spec:
+        hostAliases:
+        - hostnames:
+          - a.example.com
+          ip: 8.8.8.8
+- apiVersion: apps/v1
+  kind: Deployment
+  metadata:
+    name: deployment-b
+  spec: *hostAliases
 `),
 			exp: expected{
 				sOut: []string{`