externalSchematic executes schematics from migration.json instead of collection.json

# 🐞 Bug report

### Command

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [x] update
- [ ] lint
- [ ] extract-i18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Is this a regression?

No

### Description

The function `externalSchematic` from `@angular-devkit/schematics` does not call the schematic from the `collection.json` but instead from the `migration.json` If the collection (package) name is the same as the current package that is migrated.

## 🔬 Minimal Reproduction

1. The package `ngx-my-lib` has the schematic `config` defined in the `collection.json`
2. Create a migration schematic for the package `ngx-my-lib`.
3. Call the externalSchematic function with `externalSchematic('ngx-my-lib', 'config')` in the migration schematic.
4. Execute the migration schematic

## 🔥 Exception or Error

Schematic "config" not found in collection "ngx-my-lib"

## 🌍 Your Environment

Angular CLI: 12.0.3

Node: 14.16.1

Package Manager: yarn 1.22.10

OS: linux x64


Angular: undefined

... 

Package                      Version

------------------------------------------------------

@angular-devkit/architect    0.1200.3

@angular-devkit/core         12.0.3

@angular-devkit/schematics   12.0.3

@angular/cli                 12.0.3

@schematics/angular          12.0.3

typescript                   4.2.4

**Anything else relevant?**

I could track down this error to the `createCollection` method in the class `SchematicEngine`. Invoking the `externalSchematic` the `Collection` instance for the `migration.json` is cached with the key `ngx-my-lib`. So the external schematic execution does not load the collection for the `collection.json`, but for the `migration.json`.

SchematicEngine.createCollection と、レポートで説明されている externalSchematic の呼び出しから始め、同じパッケージ名で collection.json と migration.json がどのように読み込まれ、キャッシュされるかを追跡します。ngx-my-lib のシナリオを再現し、externalSchematic が migration.json ではなく collection.json から schematic を解決することを確認します。

索引モデルが issue の本文から書いたものです。