async doImport(importData, importOptions) {
        debug('doImport', this.importers);
        importOptions = importOptions || {};
        const importResults = {};

        for (const importer of this.importers) {
            debug('importer looking for', importer.type, 'in', Object.keys(importData));
            if (Object.prototype.hasOwnProperty.call(importData, importer.type)) {
                importResults[importer.type] = await importer.doImport(importData[importer.type], importOptions);
            }
        }

        return importResults;
    }
Import Step 3:
Each importer gets passed the data from importData which has the key matching its type - i.e. it only gets the
data that it should import. Each importer then handles actually importing that data into Ghost
@param {ImportData} importData
@param {ImportOptions} [importOptions] to allow override of certain import features such as locking a user
@returns {Promise<Object.<string, ImportResult>>} importResults