Use CompletePackageInterface in test of package

[alavieille]

In the resolution of bower dependecies, you merge rootPackages and the others packages (line 110) and test if package is an instance of CompletePackage.

But in the case where the root package uses an alias :

"branch-alias": { "dev-master": "2.0.x-dev" },
The type of package is RootAliasPackage which not extend CompletePackage like RootPackage.

So i think is better to check on an interface CompletePackageInterface to take all case

[nsams]

Thanks :)