When working with Eclipse and especially when developing plug-ins you will deal with the org.eclipse.core.resources.IProject interface.
This however does not give you an option to get the packages used in the project. For that you will need org.eclipse.jdt.core.IJavaProject interface which is a completely different interface and cannot just get it from a cast from IProject, however you can:
IProject proj = (IProject)selectionResult; IJavaProject jProj = JavaCore.create(proj);
then of course you can use jProj.getPackageFragments() to get app the package elements in the project.
You can also check if project is actually java project before creating the IJavaProject: