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

api/resmap/reswrangler_test.go

@@ -958,6 +958,53 @@ metadata:
 `), strings.TrimSpace(string(yaml)))
 }
 
+func TestDeAnchorIntoDoc(t *testing.T) {
+	input := `apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: probes-test
+spec:
+  template:
+    spec:
+      readinessProbe: &probe
+        periodSeconds: 5
+        timeoutSeconds: 3
+        failureThreshold: 3
+        httpGet:
+          port: http
+          path: /health
+      livenessProbe:
+        <<: *probe
+`
+	rm, err := rmF.NewResMapFromBytes([]byte(input))
+	assert.NoError(t, err)
+	assert.NoError(t, rm.DeAnchor())
+	yaml, err := rm.AsYaml()
+	assert.NoError(t, err)
+	assert.Equal(t, strings.TrimSpace(`apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: probes-test
+spec:
+  template:
+    spec:
+      livenessProbe:
+        failureThreshold: 3
+        httpGet:
+          path: /health
+          port: http
+        periodSeconds: 5
+        timeoutSeconds: 3
+      readinessProbe:
+        failureThreshold: 3
+        httpGet:
+          path: /health
+          port: http
+        periodSeconds: 5
+        timeoutSeconds: 3
+`), strings.TrimSpace(string(yaml)))
+}
+
 // Anchor references don't cross YAML document boundaries.
 func TestDeAnchorMultiDoc(t *testing.T) {
 	input := `apiVersion: v1

api/resource/factory_test.go

@@ -335,13 +335,7 @@ kind: List
 		"listWithAnchorReference": {
 			input:       []types.PatchStrategicMerge{patchList2},
 			expectedOut: []*Resource{testDeploymentA, testDeploymentB},
-			// The error using kyaml is:
-			//   json: unsupported type: map[interface {}]interface {}
-			// maybe arising from too many conversions between
-			// yaml, json, Resource, RNode, etc.
-			// These conversions go away after closing #3506
-			// TODO(#3271) This shouldn't have an error, but does when kyaml is used.
-			expectedErr: true,
+			expectedErr: false,
 		},
 		"listWithNoEntries": {
 			input:       []types.PatchStrategicMerge{patchList3},