Set breadcrumb only if there is a menu item for the current path.

Although its main focus is building a breadcrumb for pages that are in the menu hierarchy, menu breadcrumb sets breadcrumbs for all pages.

This may lead to conflicts with other modules that may have different bases for breadcrumb setting, particularly if the breadcrumb setting is ever moved to later in the page generation process than hook_init(). See #275622: Doesn't work for Views pages → .

To prevent this issue, menu_breadcrumb should set breadcrumbs only for paths that are in the menu hierarchy.

To facilitate this, the code for optionally appending the current page to the breadcrumb could be moved to its own function invoked late in the page generation flow, so that it would simply take any existing breadcrumb and optionally append the current page. This change would make this feature as well compatible with other breadcrumb handling modules.