do not take a link's target when removing link from tree

This causes major problems when moving links and targets between trees.
When we move the link from one tree to another, it pulls its target,
out, meaning that the target ends up becoming orphaned.

Say there is a matching link and target in two different trees, A and B,
and we want to replace the link and target in one tree with the link and
target in another.

If we move the target A to tree B first, then it replaces the target B,
and link B is updated to point to it.

Then, we move link A to tree B, replacing link B.  To do that, we move
link B out of tree B, which _takes target B with it_, leaving the ported
link A without a target in tree B.

lib/node.js

@@ -685,6 +685,7 @@ class Node {
       ...this.children.values(),
       ...this.inventory.values(),
     ].filter(n => n !== this))
+
     for (const child of family) {
       if (child.root !== root) {
         child[_delistFromMeta]()
@@ -704,12 +705,14 @@ class Node {
     }
 
     // if we had a target, and didn't find one in the new root, then bring
-    // it over as well.
-    if (this.isLink && target && !this.target)
+    // it over as well, but only if we're setting the link into a new root,
+    // as we don't want to lose the target any time we remove a link.
+    if (this.isLink && target && !this.target && root !== this)
       target.root = root
 
     // tree should always be valid upon root setter completion.
     treeCheck(this)
+    treeCheck(root)
   }
 
   get root () {

tap-snapshots/test-arborist-reify.js-TAP.test.js

@@ -1899,6 +1899,32 @@ ArboristNode {
     },
   },
   "fsChildren": Set {
+    ArboristNode {
+      "dev": true,
+      "edgesOut": Map {
+        "once" => EdgeOut {
+          "error": "MISSING",
+          "name": "once",
+          "spec": "*",
+          "to": null,
+          "type": "prod",
+        },
+        "wrappy" => EdgeOut {
+          "error": "MISSING",
+          "name": "wrappy",
+          "spec": "1.0.2",
+          "to": null,
+          "type": "prod",
+        },
+      },
+      "extraneous": true,
+      "location": "packages/a",
+      "name": "a",
+      "optional": true,
+      "path": "{CWD}/test/arborist/reify-filtered-reification-in-workspaces/packages/a",
+      "peer": true,
+      "version": "1.2.3",
+    },
     ArboristNode {
       "children": Map {
         "wrappy" => ArboristNode {

test/node.js

@@ -591,8 +591,9 @@ t.test('load with a virtual filesystem parent', t => {
   t.equal(link.root, link, 'removed from parent, removed from root')
   t.equal(root.inventory.get(linkLoc), undefined, 'removed from root inventory')
   t.equal(link.inventory.has(link), true, 'link added to own inventory')
-  t.equal(link.target, linkTarget, 'target taken along for the ride')
-  t.equal(linkTarget.root, link, 'target rooted on link')
+  t.equal(link.target, null, 'target left behind when setting root to null')
+  linkTarget.root = link
+  t.equal(link.target, linkTarget, 'target set once roots match')
   t.equal(link.inventory.get(''), linkTarget)
   t.equal(root.edgesOut.get('link').error, 'MISSING')