[MCOMPILER-544] don't add items to classpath that are not used for that

Currently simply all kind of items are added to the classpath leading to
strange errors in the ECJ compiler plugin because it does not know how
to handle pom or other possible types.

This adds an additional check to see if the artifact handler actually
claims the itme should be put on the classpath,

src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java

@@ -151,6 +151,7 @@ public class CompilerMojo extends AbstractCompilerMojo {
 
     private Map<String, JavaModuleDescriptor> pathElements;
 
+    @Override
     protected List<String> getCompileSourceRoots() {
         return compileSourceRoots;
     }
@@ -170,6 +171,7 @@ protected Map<String, JavaModuleDescriptor> getPathElements() {
         return pathElements;
     }
 
+    @Override
     protected File getOutputDirectory() {
         File dir;
         if (!multiReleaseOutput) {
@@ -180,6 +182,7 @@ protected File getOutputDirectory() {
         return dir;
     }
 
+    @Override
     public void execute() throws MojoExecutionException, CompilationFailureException {
         if (skipMain) {
             getLog().info("Not compiling main sources");
@@ -344,11 +347,14 @@ private List<File> getCompileClasspathElements(MavenProject project) {
         list.add(new File(project.getBuild().getOutputDirectory()));
 
         for (Artifact a : project.getArtifacts()) {
-            list.add(a.getFile());
+            if (a.getArtifactHandler().isAddedToClasspath()) {
+                list.add(a.getFile());
+            }
         }
         return list;
     }
 
+    @Override
     protected SourceInclusionScanner getSourceInclusionScanner(int staleMillis) {
         if (includes.isEmpty() && excludes.isEmpty() && incrementalExcludes.isEmpty()) {
             return new StaleSourceScanner(staleMillis);
@@ -363,6 +369,7 @@ protected SourceInclusionScanner getSourceInclusionScanner(int staleMillis) {
         return new StaleSourceScanner(staleMillis, includes, excludesIncr);
     }
 
+    @Override
     protected SourceInclusionScanner getSourceInclusionScanner(String inputFileEnding) {
         // it's not defined if we get the ending with or without the dot '.'
         String defaultIncludePattern = "**/*" + (inputFileEnding.startsWith(".") ? "" : ".") + inputFileEnding;
@@ -375,10 +382,12 @@ protected SourceInclusionScanner getSourceInclusionScanner(String inputFileEndin
         return new SimpleSourceInclusionScanner(includes, excludesIncr);
     }
 
+    @Override
     protected String getSource() {
         return source;
     }
 
+    @Override
     protected String getTarget() {
         return target;
     }
@@ -388,14 +397,17 @@ protected String getRelease() {
         return release;
     }
 
+    @Override
     protected String getCompilerArgument() {
         return compilerArgument;
     }
 
+    @Override
     protected Map<String, String> getCompilerArguments() {
         return compilerArguments;
     }
 
+    @Override
     protected File getGeneratedSourcesDirectory() {
         return generatedSourcesDirectory;
     }