Improvements to farm_migrate_role migration config.

These changes help to support custom/contrib modules that want
to provide their own role migrations.

- Use skip_map_on_empty process plugin to ensure that skipped rows are not added to the map tables.
- Set a "role" pseudo-field, which is used by role ID matching.
- Move role ID matching process to farm_migrate_role migration group shared config.
- Move farm_migrate_role migration to optional config, dependent on farm_role_roles module.

modules/core/migrate/config/install/migrate_plus.migration_group.farm_migrate_role.yml

@@ -9,4 +9,24 @@ label: 'farmOS 1.x Role Migration'
 description: 'Migrates roles from farmOS 1.x to farmOS 2.x'
 source_type: 'farmOS 1.x'
 module: null
-shared_configuration: null
+shared_configuration:
+  process:
+    # This role field is a placeholder, and should be replaced in the specific
+    # migration process configuration. This makes it easier for custom/contrib
+    # modules to provide role migrations. See the farm_migrate_user migration
+    # for an example that can be copied.
+    role:
+      - plugin: default_value
+        default_value: false
+    id:
+      -
+        plugin: get
+        source: '@role'
+      -
+        plugin: entity_lookup
+        value_key: id
+        entity_type: user_role
+        ignore_case: true
+      -
+        plugin: skip_map_on_empty
+        method: row

modules/core/migrate/config/optional/migrate_plus.migration.farm_migrate_role.yml

@@ -4,6 +4,7 @@ dependencies:
   enforced:
     module:
       - farm_migrate
+      - farm_role_roles
 id: farm_migrate_role
 label: 'Roles'
 migration_group: farm_migrate_role
@@ -18,22 +19,14 @@ source:
 destination:
   plugin: 'entity:user_role'
 process:
-  id:
-    -
-      plugin: static_map
-      source: name
-      map:
-        Farm Manager: farm_manager
-        Farm Worker: farm_worker
-        Farm Viewer: farm_viewer
-    -
-      plugin: entity_lookup
-      value_key: id
-      entity_type: user_role
-      ignore_case: true
-    -
-      plugin: skip_on_empty
-      method: row
+  role:
+    plugin: static_map
+    source: name
+    map:
+      Farm Manager: farm_manager
+      Farm Worker: farm_worker
+      Farm Viewer: farm_viewer
+    bypass: true
 migration_dependencies:
   required: {  }
   optional: {  }