@Test
    public void imported_fields_are_enumerated_in_document_type() {
        var docType = getBook().getDataType();
        assertEquals(2, docType.getImportedFieldNames().size());
        assertTrue(docType.hasImportedField("my_dummy"));
        assertTrue(docType.hasImportedField("my_foo"));
        assertFalse(docType.hasImportedField("some_field_that_does_not_exist"));
    }
Packs the given doc to Book
@param d doc, never null
@return a Book object or null if input doc isn't a Book
documentgen-test/src/test/java/com/yahoo/vespa/config/DocumentGenPluginTest.java