Fix "m2e plugin sometimes 'loses' resources" (eclipse-m2e#1511)

The Java Builder may delete files from the project output directory that
need to be re-created by the m2e Maven Builder.

With commit 8e5cd49 (a fix for eclipse-m2e#1275),
any changes to the project output directory were ignored, leading to
unexpected errors when running an application after modifying the
project POM: resources were missing from the target classpath, leading
all sorts of unexpected program behavior.

This change allows marking these changes as relevant, as long as the
following conditions are met:

- The expected resource no longer exists (i.e., it was deleted by
  another builder, plugin or outside process)
- The modification applies to a resource in the classes or test-classes
  folder (i.e., outputLocation/testOutputLocation)

eclipse-m2e#1511
eclipse-m2e#1275

org.eclipse.m2e.core/src/org/eclipse/m2e/core/internal/builder/MavenBuilderImpl.java

@@ -209,6 +209,10 @@ private boolean hasRelevantDelta(IMavenProjectFacade projectFacade, IResourceDel
     if(project == null || buildOutputLocation == null) {
       return true;
     }
+
+    IPath outputLocation = projectFacade.getOutputLocation();
+    IPath testOutputLocation = projectFacade.getTestOutputLocation();
+
     IPath projectPath = project.getFullPath();
     List<IPath> moduleLocations = projectFacade.getMavenProjectModules().stream()
         .map(module -> projectPath.append(module)).toList();
@@ -219,7 +223,12 @@ private boolean hasRelevantDelta(IMavenProjectFacade projectFacade, IResourceDel
         IPath fullPath = delta.getFullPath();
         if(buildOutputLocation.isPrefixOf(fullPath)) {
           //anything in the build output is not interesting for a change as it is produced by the build
-          //lets see if there are more interesting parts...
+          // ... unless a classpath resource that existed before has been deleted, possibly by another builder
+          if(!resource.exists() && ((outputLocation != null && outputLocation.isPrefixOf(fullPath))
+              || (testOutputLocation != null && testOutputLocation.isPrefixOf(fullPath)))) {
+            hasRelevantDelta.set(true);
+            return false;
+          }
           return true;
         }
         for(IPath modulePath : moduleLocations) {