[core] Rename create/destroyProxy() of AbstractImporter to reflect their real use

The methods createProxy() and destroyProxy() has been renamed
useImportDeclaration() and denyImportDeclaration().

The fact is that the Importer doesn't not necessarily have to create a proxy
with the given ImportDeclaration. He can just publish an other
ImportDeclaration, create a proxy and publish and other ImportDeclaration or do
nothing.