Ensure P2P init hook was invoked before indexing.

Fix a case where P2P connexions are not registered when trying to
index P2P facets. This is due to FacetWP ajax handler running its
actions on "init" hook which come before P2P has the chance of running
its action "p2p_init".

A check has been added in the two custom index processor to trigger
the P2P hook if it didn't already run.

facetwp-p2p.php

@@ -242,6 +242,11 @@ public function p2p_indexer( $bypass, $defaults ) {
 			return $bypass;
 		}
 
+		// Ensure P2P init hook was invoked.
+		if ( ! did_action( 'p2p_init' ) ) {
+			do_action( 'p2p_init' );
+		}
+
 		$connexion      = $source[1];
 		$connexion_side = $source[2];
 		$post_ptype     = get_post_type( (int) $params['post_id'] );
@@ -299,6 +304,11 @@ public function p2pmetas_indexer( $bypass, $defaults ) {
 			return $bypass;
 		}
 
+		// Ensure P2P init hook was invoked.
+		if ( ! did_action( 'p2p_init' ) ) {
+			do_action( 'p2p_init' );
+		}
+
 		$connexion  = $source[1];
 		$field_name = $source[2];
 		$post_id    = (int) $params['post_id'];