fixes a CtPackage problem

This pull request fixes a problem related to the packages. 
A package which contains a package-info.java file doesn't have a SourceReference instance. This pr adds the information of the SourceReference to the package if a package-info.java file exists.
Furthermore if an CtElement doesn't have a SourceReference info, the implementation tries to load the information of the parent element. This doesn't work for packages. The parent element of a package is another package and it's package-java.file isn't related to the current package. Therefore I override the method within the CtPackageImpl, which just returns the own SourcePosition.