If mainClass not specified, it will find main classes not belonging to current workspace.  #1098

Also see: microsoft/vscode-spring-boot-dashboard#152

Steps to repro:

1. open a spring boot project.
2. [important]open a .java file containing a main function, e.g. App.java, which is outside this workspace.
3. close this .java file.
4. in launch.json, create a java launch config, keep projectName but remove mainClass field.
5. Press F5, it searches for main class to run.

Current behavior

It pops up a list of main class, including App and the expected spring boot main class.

Expected behavior

Directly launch spring boot project's main class.

Culprit is probably that the App.java was added into the hidden default project.