Standardize source module discovery mechanism

In general I think this looks good, i'll review the new keys in core independently.

+++ b/src/Form/MigrateUpgradeForm.php
@@ -742,12 +245,20 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
+      if (isset($source['provider']) && $destination['provider']) {
+        $source_module = $source['provider'];
+        $destination_module = $destination['provider'];
...
+      else {
+        $source_module = $this->t('Unknown');
+        $destination_module = $this->t('Unknown');

If you have a source provider but not a destination provider, you'll still end up with Unknown for both.