When building the dependency graph, maven modules which are not buildable should be ignored

Currently only modules which are disabled themselves - i.e. are no longer part of the moduleset - are excluded.
We should also exclude modules which are not buildable - i.e. whose MavenModuleSets are disabled.

See MavenModule#buildDependencyGraph()
for (MavenModule m : Hudson.getInstance().getAllItems(MavenModule.class)) {
if(m.isDisabled()) continue;

should be replaced with
for (MavenModule m : Hudson.getInstance().getAllItems(MavenModule.class)) {
if(!m.isBuildable()) continue;