Improve ResolveMainClassHandler.resolveMainClassUnderPaths

[snjeza]

Steps to reproduce:

• clone and import the spring-boot project - See Improve ProjectCommand.getMainClasses vscode-java-dependency#882 (comment)
• call the Debug action on the spring-boot project in the Java Projects view

The action takes 5+ seconds on my machine.
Java Debugger searches for the main method in all the projects instead of the spring-boot project.
This method is called for each project if the Spring Boot Dashboard extension is installed.
If we search only the spring-boot project, the action takes <200ms

I have created

• https://github.com/snjeza/vscode-test/raw/refs/heads/master/vscode-java-debug-0.58.3.vsix
• https://github.com/snjeza/vscode-test/raw/refs/heads/master/vscode-java-dependency-0.24.2.vsix
• https://github.com/snjeza/vscode-test/raw/refs/heads/master/vscode-spring-boot-dashboard-0.14.1.vsix

for testing.