Make sure extra source roots are used in gwt:devmode and gwt:codeserver

Such extra source roots aren't uncommon when using annotation processing,
or any other code generator.

Because the lifecycle is forked by those goals, they have to use the
forked "execution projects" rather than ones from the "initial" reactor.

Fixes #27

src/main/java/net/ltgt/gwt/maven/AbstractDevModeMojo.java

@@ -145,6 +145,9 @@ public void execute() throws MojoExecutionException, MojoFailureException {
 
     LinkedHashSet<String> sources = new LinkedHashSet<>();
     for (MavenProject p : projectList) {
+      if (p.getExecutionProject() != null) {
+        p = p.getExecutionProject();
+      }
       addSources(p, sources);
     }