feat: omit resolved from registry dependencies

Start npm/rfcs#486. This implements `$disable-write-resolves` without
creating an option. Next I'll figure out how to plumb a npm config
option thru to shrinkwrap.

lib/node.js

@@ -508,6 +508,14 @@ class Node {
     return this === this.root || this === this.root.target
   }
 
+  get isRegistryDependency () {
+    if (this.edgesIn.size === 0) return false
+    for (const edge of this.edgesIn) {
+      if (!npa(edge.spec).registry) return false
+    }
+    return true
+  }
+
   * ancestry () {
     for (let anc = this; anc; anc = anc.resolveParent) {
       yield anc

lib/shrinkwrap.js

@@ -284,7 +284,7 @@ class Shrinkwrap {
     })
 
     const resolved = consistentResolve(node.resolved, node.path, path, true)
-    if (resolved) {
+    if (resolved && !node.isRegistryDependency) {
       meta.resolved = resolved
     }
 
@@ -953,6 +953,7 @@ class Shrinkwrap {
     // git, file, dir, or remote, then the resolved value is necessary.
     if (node.resolved &&
         !node.isLink &&
+        !node.isRegistryDependency &&
         rSpec.type !== 'git' &&
         rSpec.type !== 'file' &&
         rSpec.type !== 'directory' &&