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/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},