Failed to resolve classpath: Main class 'Single' isn't unique in the workspace, please pass in specified projectname.

Repro steps:
1. open a normal console application like 'simpleapp'
2. Open a single java file named `Single.java`  with main class
3. F5 to generate launch.json
4. F5 again, in the main class select Single,
5. the error dialog shows:
![image](https://user-images.githubusercontent.com/17075863/45399087-68c77400-b679-11e8-90fe-4be8754d06e7.png)


