Process plugin for importing/creating terms by name

Possibly we could use something akin to https://www.drupal.org/node/2181783, but a little more specific for term entity reference fields.

This is simply a WIP. Marking postponed, since we have two dependencies that need to land first.

#2633428: Add a Case Sensitive Condition to Entity Query is no longer a dependency. D7 didn't do the case comparison in a SQL query, probably because it isn't possible there either. We'll have to retrieve results, then do the comparison in PHP. The getter issue is also seeing good progress.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,139 @@
   +  /** @var \Drupal\Core\Entity\EntityStorageInterface */
   +  protected $migrationStorage;
   ...
   +    $this->migrationStorage = $entity_manager->getStorage('migration');
   

   No longer necessary.

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,139 @@
   +    $bundle = $this->getBundle();
   +    $config_target = $this->entityManager->getStorage('field_config')->load($entity_type . '.' . $bundle . '.' . $destination_property);
   

   No longer necessary.

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,139 @@
   +    $vid = reset($config_target->getSetting('handler_settings')['target_bundles']);
   

   We should provide the vid from configuration instead. And throw an exception if it doesn't provide one.

4. +++ b/core/modules/migrate/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,139 @@
   +  protected function getBundle() {
   +    return 'foo';
   +  }
   

   This is unnecessary.

1. +++ b/modules/contrib/migrate_plus/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,123 @@
   +    $vid = $this->configuration['vid'];
   +
   ...
   +    if (!isset($this->configuration['vid'])) {
   

   This isset check should happen before we grab the value from configuration. In case it isn't set...

2. +++ b/modules/contrib/migrate_plus/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,123 @@
   +    //Verify if vocabulary id is not empty.
   

   Nit: spaces between // and the sentence.

3. +++ b/modules/contrib/migrate_plus/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,123 @@
   +    //Verify if vocabulary is exist. .
   

   Nit: Only a single period is necessary.

4. +++ b/modules/contrib/migrate_plus/src/Plugin/migrate/process/TermReference.php
   @@ -0,0 +1,123 @@
   +

    if (is_null(Vocabulary::load($vid))) {
   

   Nit: this can just be:

   if (!Vocabulary::load($vid)) {

And I guess we should create some unit tests for this.

re #13, it does solve part of the problem. But it doesn't create a stub.

Yes, I'm all for offloading the search to the other class. But then we'd want to make as few assumptions there as possible. I noticed you mentioned we'd want bundle, but we really don't need that for terms reference. We just need the destination vid. But making a default_value, "required" seems a little strong-handed. There's already a default value plugin. Use it instead.

re: #20, that assumes this is a d2d migration where there is already a separate term migration. But in D7 contrib migrate and in my case on D8, I'm migrating in from CSV. There's 4 taxonomy reference fields on the node. I don't want to create 4 additional migrations. Ideally, I can read over the CSV once and insert all the term stubs at the same time as I'm creating the node. From an I/O perspective, CSV isn't all that performant. So reducing the number of times I have to process the same text file significantly cuts down on processing time.

Cleanup after making some updates to #2642612: Create Entity Lookup & Generate process Base plugins

This won't be necessary after #2642612: Create Entity Lookup & Generate process Base plugins is completed.