Rename *.menu_links.yml to *.links.menu.yml files to improve DX

Postponed on #2256521: [META] New plan, Phase 2: Implement menu links as plugins, including static admin links and views, and custom links with menu_link_content entity, all managed via menu_ui module.

Does this really need to be postponed on all 5 parts of the menu link conversion? #2301239: MenuLinkNG part1 (no UI or conversions): plugins (static + MenuLinkContent) + MenuLinkManager + MenuTreeStorage already landed, and I don't think this patch conflicts with any of the other parts.

Ah! If it doesn't conflict with the rest of the patches, then by all means.

Cool. Back to needs review then. On dreditor inspection, patch looks good to me, but I haven't checked if it's complete or reviewed the change record, so leaving to others to RTBC.

There aren't other menu links left:

$ find . -name "*menu_links.yml"
./modules/webprofiler/webprofiler.menu_links.yml
$ git grep "menu_links.yml" .

One thing we should talk about in a follow up, the hook names no longer match the file names at all:

• hook_menu_link_defaults_alter
• hook_menu_local_tasks_alter
• hook_menu_local_actions_alter
• hook_contextual_links_alter

See #2304067: Adapt menu alter hooks to the new filenames

Committed b560f34 and pushed to 8.x. Thanks!