Update menu link definitions when aliases change

Waiting for: https://www.drupal.org/node/2417877 but rolling that patch in together for now.

Re-roll for patch committed at #2417877: Make getDefinition method on menu link content entity public so we can rebuild user paths

and fix missing "_"

Needs to handle alias deletion and add tests, at least.

Marking critical, since it's going to be a regression in behavior otherwise.

needed to rename other function calls.

@webchick - it only didn't break until someone triggered a menu rebuild (menu link discovery), so this avoids a greater WTF of having it still work for while and then break.

Looks like painful, but good, progress. Some small feedback:

1. +++ b/core/modules/menu_link_content/menu_link_content.install
   @@ -0,0 +1,15 @@
   +  // system_path_uodate().
   

   typo in function name

2. +++ b/core/modules/menu_link_content/menu_link_content.module
   @@ -36,3 +36,54 @@ function menu_link_content_menu_delete(MenuInterface $menu) {
   +  /* @var $entities  \Drupal\menu_link_content\MenuLinkContentInterface[] */
   

   needs to be reformatted

3. +++ b/core/modules/menu_link_content/menu_link_content.module
   @@ -36,3 +36,54 @@ function menu_link_content_menu_delete(MenuInterface $menu) {
   +  if ($path['source'] != $path['original']['source']) {
   +    _menu_link_content_update_path_alias($path['alias']);
   +    _menu_link_content_update_path_alias($path['source']);
   +    _menu_link_content_update_path_alias($path['original']['source']);
   

   Should this be elseif() ?

   Do we need to run the helper on the source and original source also?

4. +++ b/core/modules/menu_link_content/src/Tests/PathAliasMenuLinkContentTest.php
   @@ -0,0 +1,89 @@
   +    module_set_weight('menu_link_content', 1);
   

   Maybe a todo on whether we could remove this hack if we cache the UrlGenerator and stop caching path aliases?

+++ b/core/modules/menu_link_content/src/Tests/PathAliasMenuLinkContentTest.php
@@ -0,0 +1,89 @@
+      'link' => ['uri' => 'user-path:destination'],

this path is sort of confusing (at least when I'm also looking at the issue about the destination query string). Fixing that too.

considering a pathauto-like URI like: user-path:content/about-us

The more variable part is only after 18 characters, so a 25 or 30 char index seems more reasonable.