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

从 SchematicEngine.createCollection 和报告中描述的 externalSchematic 调用开始，然后跟踪 collection.json 和 migration.json 如何以相同的包名加载并缓存。复现 ngx-my-lib 场景，并验证 externalSchematic 解析的是 collection.json 中的 schematic，而不是 migration.json 中的 schematic。

🐞 Bug report

Command

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

Is this a regression?

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.