Deprecate FacetapiAdapter::getFacetQuery() in favor of a method names FacetapiAdapter::getQueryType()

Once upon a time we could not use the FacetapiAdapter::getQueryType() method to get a facet's query type plugin because it was being used for something else in places that couldn't be changed. Those times are gone, so we should standardize on nomenclature for methods returning plugin instances and deprecate FacetapiAdapter::getFacetQuery() in favor of a method named FacetapiAdapter::getQueryType().