Improve ProjectCommand.getMainClasses

[snjeza]

Steps to reproduce:

• clone the spring-boot project

$ git clone git@github.com:spring-projects/spring-boot.git
$ cd spring-boot
$ ./gradlew publishToMavenLocal

• set a breakpoint at

  vscode-java-dependency/jdtls.ext/com.microsoft.jdtls.ext.core/src/com/microsoft/jdtls/ext/core/ProjectCommand.java

  Line 249 in ee5172f

  IJavaSearchScope scope = SearchEngine.createJavaSearchScope(searchRoots.toArray(new IJavaElement[0]));

$ code .

ProjectCommand.getMainClasses creates 20000+ IPackageFragmentRoot objects in the searchRoots variable.
Only sources in the Java projects can be searched.
The related issue - #811