Implement hook_hook_info() to allow field type modules to offload migration code into include files

hook_hook_info() allows modules that "own" a hook to specify that other modules may put the hook implementations into include files that are loaded on demand.

This especially makes sense for hook_content_migrate_*() hooks, which are needed once only.

You can see an optimized implementation of hook_hook_info() in #1165388-7: Add mollom_hook_info() to offload Mollom hooks into modulename.mollom.inc:

 /**
+ * Implements hook_hook_info().
+ */
+function mollom_hook_info() {
+  $hooks = array(
+    'mollom_form_list',
+    'mollom_form_list_alter',
+    'mollom_form_info',
+    'mollom_form_info_alter',
+    'mollom_form_insert',
+    'mollom_form_update',
+    'mollom_form_delete',
+    'mollom_data_insert',
+    'mollom_data_update',
+    'mollom_data_delete',
+  );
+  $hooks = array_fill_keys($hooks, array(
+    'group' => 'mollom',
+  ));
+  return $hooks;
+}