Fix NarTestCompileMojo to exclude executable dependency

Improve NarAssemblyMojo performance by skip copying older files

NarTestCompileMojo will be failed if dependent project is an executable. The fix
is simply by-passing the dependent executable projects. NarCompileMojo has the
same logic to filter executable projects.

NarAssemblyMojo improve performance by calling
org.codehaus.plexus.util.FileUtils.copyDirectoryStructureIfModified to skip
copying files with old timestamps.