ts版 引用自定义框架 设置 "declarations": true ，启动后报 AssertionError [ERR_ASSERTION]: Symbol.for('egg#eggPath') is required on Application
## What happens?
A clear and concise description of what the bug is.
ts版 引用自定义框架 设置 "declarations": true ，启动后报
 AssertionError [ERR_ASSERTION]: Symbol.for('egg#eggPath') is required on Application

![image](https://user-images.githubusercontent.com/10991856/97385686-2cb10500-190d-11eb-8d4c-f101b898f6fc.png)

![image](https://user-images.githubusercontent.com/10991856/97385655-12772700-190d-11eb-9faf-dbf7acfb632f.png)


## Mini Showcase Repository(REQUIRED)
> Provide a mini GitHub repository which can reproduce the issue.
> Use `npm init egg --type=simple bug` then upload to your GitHub

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## How To Reproduce

**Steps to reproduce the behavior:**
1.使用 npm init egg --type=ts 搭建demo
2.npm link examples/framework/yadan
3.npm run debug

**Expected behavior**
1.
2.

## Context
- **Node Version**:v10.16.3
- **Egg Version**: ^2.6.1
- **Plugin Name**:
- **Plugin Version**:
- **Platform**:
