Support launch without pom.xml

This closes #1427

Author: kwin

org.eclipse.m2e.launching/META-INF/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %Bundle-Name
 Bundle-SymbolicName: org.eclipse.m2e.launching;singleton:=true
-Bundle-Version: 2.0.500.qualifier
+Bundle-Version: 2.0.600.qualifier
 Bundle-Localization: plugin
 Require-Bundle: org.eclipse.core.runtime,
  org.eclipse.core.variables,

org.eclipse.m2e.launching/src/org/eclipse/m2e/internal/launch/MavenLaunchDelegate.java

@@ -35,6 +35,7 @@
 import org.eclipse.core.resources.ResourcesPlugin;
 import org.eclipse.core.runtime.CoreException;
 import org.eclipse.core.runtime.ILog;
+import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProgressMonitor;
 import org.eclipse.core.runtime.NullProgressMonitor;
 import org.eclipse.core.runtime.Path;
@@ -244,14 +245,17 @@ public static String readEnforcedJavaVersion(File pomDirectory, IProgressMonitor
     try {
       Optional<IContainer> container = getContainer(pomDirectory);
       if(container.isPresent()) {
-        IMavenProjectRegistry projectManager = MavenPlugin.getMavenProjectRegistry();
-        IFile pomFile = container.get().getFile(Path.fromOSString(IMavenConstants.POM_FILE_NAME));
-        IMavenProjectFacade mavenProject = projectManager.create(pomFile, true, new NullProgressMonitor());
-        if(mavenProject != null) {
-          return readEnforcedVersion(mavenProject, monitor);
+        IPath pomPath = Path.fromOSString(IMavenConstants.POM_FILE_NAME);
+        if(container.get().exists(pomPath)) {
+          IFile pomFile = container.get().getFile(pomPath);
+          IMavenProjectRegistry projectManager = MavenPlugin.getMavenProjectRegistry();
+          IMavenProjectFacade mavenProject = projectManager.create(pomFile, true, new NullProgressMonitor());
+          if(mavenProject != null) {
+            return readEnforcedVersion(mavenProject, monitor);
+          }
         }
       }
-      //TODO: handle the case if the pomDirectory points to a project not in the project. Then load the bare project.
+      //TODO: handle the case if the pomDirectory points to a project not in the workspace. Then load the bare project.
     } catch(CoreException ex) {
       logEnforcedJavaVersionCalculationError(ex);
     }