Impossible to enable views if entities are not in SQL: part 2

Problem/Motivation

Standard doesn't install for MongoDB cos taxonomy_views_data_alter makes $data['node_field_data'] exist.

Proposed resolution

View::isInstallable should check whether the base table is a base table (OK that sounds stupid but that's what's needed).

Note there is no test. The isInstallable mechanism has a test but writing a direct test against this would be so directly testing the implementation itself that it's completely pointless. For a real test, I recommend https://events.drupal.org/node/605 .