fix(@angular/cli): ng-update migrations not running with --migrate-only

With Angular CLI version 8, migrations cannot be re-run with the
`--migrate-only` flag as there was a recent regression introduced
in e406f00#diff-0d0a748fb9a38a7ccde08d9b42e70bce as it now passes
a normalized platform path to the `engine.createCollection` call.

This breaks as there is incorrect logic within
`node-modules-engine-host` that causes the schematic collection to be
searched within the `package.json#schematics` entry. This is incorrect
as migration schematics specify their migration schematics in a separate
schematic collection file which is part of `package.json#ng-update`.

Fixes #14565

packages/angular_devkit/schematics/tools/node-module-engine-host.ts

@@ -79,7 +79,7 @@ export class NodeModulesEngineHost extends FileSystemEngineHostBase {
   protected _resolveCollectionPath(name: string): string {
     let collectionPath: string | undefined = undefined;
 
-    if (name.replace(/\\/, '/').split('/').length > (name[0] == '@' ? 2 : 1)) {
+    if (name.replace(/\\/g, '/').split('/').length > (name[0] == '@' ? 2 : 1)) {
       try {
         collectionPath = this._resolvePath(name, process.cwd());
       } catch {