[schematics] more details about the missing fields

Error: Schematic "init" is missing fields.
what exactly the missing fields?

```
//collection.json
{
"$schema": "../../../node_modules/@angular-devkit/schematics/collection-schema.json",
"schematics": {
"init": {
"description": "creates a new builder",
"factory": "./index#init",
"schema": "./init.schema.json"
}
}
}
```
modify:
after houres of inspecting, the issue was from another schematic ( i.e: using externalSchematic() )
so, it is better to explain whitch schematic that this issue belongs to

先从解析 `collection.json` 中的 `init` 条目时涉及的 Schematics 错误路径开始，然后重现 `externalSchematic()` 调用另一个 schematic 的情况。跟踪 `init.schema.json` 和外部 schematic 的识别过程；当错误信息指出缺少字段所属的 schematic 时，即表示完成。

由索引模型根据 Issue 内容生成。