Extract conflict detection to it's own interface.

This PR
 - defines a patch conflict detector interface,
 - extracts implementations of the interface from the
   merginator code, making the merginator code
   independent of --enable_kyaml.
 - injects those implementations into kustomize
   as a function of --enable_kyaml.

So, instead of using different merginators to combine
resmaps, this pr allows the use of a single patch merge
code path that uses different conflict detectors.

So instead of debating how to merge, we're now only
considering whether to warn on conflict detection
in one transformer.

This PR is in service of #3304, eliminating seven
instances where --enable_kyaml was consulted.  These
were cases where conflict detection wasn't an issue
(but merging patches was).

api/resource/conflictdetector.go

@@ -0,0 +1,20 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package resource
+
+import "sigs.k8s.io/kustomize/api/resid"
+
+// ConflictDetector detects conflicts between resources.
+type ConflictDetector interface {
+	// HasConflict returns true if the given resources have a conflict.
+	HasConflict(patch1, patch2 *Resource) (bool, error)
+	// Merge two resources into one.
+	MergePatches(patch1, patch2 *Resource) (*Resource, error)
+}
+
+// ConflictDetectorFactory makes instances of ConflictDetector that know
+// how to handle the given Group, Version, Kind tuple.
+type ConflictDetectorFactory interface {
+	New(gvk resid.Gvk) (ConflictDetector, error)
+}