Remove @internal from d7/source/Field.php

Problem/Motivation

In working on Ubercart 7 to Commerce 2 I came across a reason to extend Field.php but it is marked @internal. The reason stems from the fact that products are nodes in Ubercart 7 but commerce_products in Commerce 2. If a field exists on a product node and another non product node type the Commerce 2 sites needs field storage on the node entity and the commerce_product entity. To do this adding a new row to the field plugin mimicking that the field existed in the source on a commerce_product entity works well.

Field.php was marked internal in #2891932: Refactor D7 Field source plugin and the references to the decision are in comments in #10 and #11. It states that @internal was used to keep some flexibility if this needed to be changed in the future.

The plugin was refactored in 8.4.x, there are have been 3 changes to the file since then, 2 for fixes to the annotation and 1 to add handling of Drupal 7 title data.

Proposed resolution

Since nothing has turned up yet to indicate that this needs to remain internal, let's remove it.

Remaining tasks

Patch that removes the following for d7/Field.php

 * @internal
 *
 * This class is marked as internal and should not be extended. Use
 * Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase instead.
 *

Review
Commit

API changes

The source plugin is no longer @internal.

Marking novice for making the initial patch as described in 'Remaining Tasks'.