Avoid central index.cds files to ease extensibility

fiori/db/schema.cds

@@ -0,0 +1,8 @@
+using { sap.capire.bookshop } from '@capire/bookshop';
+
+// Forward-declare calculated fields to be filled in database-specific ways
+// TODO find a better way to have 'default' fields that still can be overwritten.
+extend bookshop.Authors with {
+	virtual age: Integer;
+	virtual lifetime: String;
+}