Plugin to only migrate users with certain roles

This adds a new source plugin called d7_user_role_filter which looks for a "roles" value in the yml file. Here's the docblock:

/**
 * Drupal 7 user source from database that filters based upon role(s).
 *
 * Requires the "roles" annotation value; see examples below.
 *
 * @MigrateSource(
 *   id = "d7_user_role_filter",
 *   source_module = "user"
 * )
 *
 * Examples:
 *
 * Example usage that requires users to have role rid values 3, 5.
 * @code
 * source:
 *   plugin: d7_user_role_filter
 *   roles:
 *     - 3
 *     - 5
 * @endcode
 */

Thanks to the folks in the #migration channel on Slack for their help, including ttamniwdoog, benjifisher, timwood, heddn and mikelutz.

While this doesn't work on the changes requested, ran out of time today, this adds *another* plugin that also adds all users who have created nodes, though I need to test & tweak it.

Fixed the UserAuthorRoleFilter and changed UserRoleFilter::query() so it extends the base query instead of duplicating the code.

Gah, mucked up the last patches, sorry.

Going over the feedback from #4:

• From what I could see, "skip_on_value" would only work on the source data, and the d7_user migration plugin doesn't pull in the roles data (or the node_revisions data) so it wasn't going to work.
• I agree that it would be better to filer based upon the role name, I just didn't do that extra step yet.
• An option to negate the filter would also be good.

This renames the class variable, changes the variable check to a cast statement, removes a commented out print_r() statement, and leaves two @todo items for later.

The UNION statement isn't working as intended because SqlBase::initializeIterator() is joining a extra table to the first query, but not the second one, so it throws this error:

Cardinality violation: 1222 The used SELECT statements have a different number of columns

SELECT DISTINCT u.*
FROM {users} u
INNER JOIN {node_revision} nr ON u.uid = nr.uid
WHERE (u.uid > :db_condition_placeholder_0)

and turning it into this:

SELECT DISTINCT u.*, map.sourceid1 AS migrate_map_sourceid1,
map.source_row_status AS migrate_map_source_row_status
FROM {users} u
INNER JOIN {node_revision} nr ON u.uid = nr.uid
LEFT OUTER JOIN SOURCEDB.migrate_map_PLUGINNAME map ON u.uid = map.sourceid1
WHERE (u.uid > :db_condition_placeholder_0) AND ((map.sourceid1 IS NULL) OR (map.source_row_status = :db_condition_placeholder_1))

I split this up into two separate requests, so this one is back to just the user role filter.

This needed a $query->distinct(TRUE) to make sure the query count was correct.

Glad it worked for you, @patpluspun.

Let's go back to the request to allow limiting it by role name.