Use src/main/java when generating TemplateExtensions.java

Hard code the folder that `TemplateExtensions.java` is generated in
to `./src/main/java`,
and throw an exception if that path isn't on the classpath to indicate
that the quick fix failed.

We will need to improve this in the future,
since the location of the default package can be configured
in Maven and Gradle.

Fixes redhat-developer#831

Signed-off-by: David Thompson <davthomp@redhat.com>

qute.jdt/com.redhat.qute.jdt/src/main/java/com/redhat/qute/jdt/internal/template/TemplateGenerateMissingJavaMember.java

@@ -11,16 +11,22 @@
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
+import java.util.Optional;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.stream.Stream;
 
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProgressMonitor;
+import org.eclipse.core.runtime.Path;
 import org.eclipse.jdt.core.Flags;
 import org.eclipse.jdt.core.ICompilationUnit;
 import org.eclipse.jdt.core.IField;
+import org.eclipse.jdt.core.IJavaElement;
 import org.eclipse.jdt.core.IJavaProject;
 import org.eclipse.jdt.core.IPackageFragment;
 import org.eclipse.jdt.core.IPackageFragmentRoot;
@@ -378,9 +384,25 @@ private static WorkspaceEdit createNewTemplateExtensionFile(GenerateMissingJavaM
 
 		IPackageFragment destPackage = null;
 		try {
+			IPath sourceFilesPath = project.getResource().getProjectRelativePath().append("src/main/java");
+			Optional<IPackageFragmentRoot> sourceFragmentRootOptional = Stream.of(project.getPackageFragmentRoots()) //
+					.filter(pfr -> {
+						try {
+							if (pfr.getKind() == IPackageFragmentRoot.K_BINARY || pfr.getResource() == null) {
+								return false;
+							}
+							return pfr.getResource().getProjectRelativePath().equals(sourceFilesPath);
+						} catch (JavaModelException e) {
+							return false;
+						}
+					}) //
+					.findFirst();
+			if (sourceFragmentRootOptional.isEmpty()) {
+				throw new UnsupportedOperationException("This quick fix only works for projects where the code is stored in src/main/java");
+			}
+			IPackageFragmentRoot fragmentRoot = sourceFragmentRootOptional.get();
 			// TODO: longest substring match of package of original class
 			// OR: figure out the "correct" one from the groupId?
-			IPackageFragmentRoot fragmentRoot = project.getPackageFragmentRoots()[0];
 			destPackage = fragmentRoot.getPackageFragment("");
 		} catch (JavaModelException e) {
 		}