[FIX] website: do not suggest generic pages for existing specific ones

Before this commit, when obtaining link URL suggestions, both the
specific and the matching generic page were suggested.

After this commit, only the most specific ones are kept in the suggested
list.
This commit also adapts the sitemap in the same way.
In stable, a condition on a dedicated context key is used in case those
methods were called with the goal of obtaining both generic and specific
pages.
In 16.0, those methods will always filter duplicates pages as it was
supposed at first.

Steps to reproduce:
- Edit Contact Us page (to create a specific view)
- Edit the Contact Us menu
- Type "/" in the URL
=> "/contactus" appeared twice.

task-2968292

closes odoo/odoo#112746

X-original-commit: c5a50362ef55ced373c53c7af069fd5534043ae2
Signed-off-by: Romain Derie (rde) <rde@odoo.com>
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Benoit Socias (bso) <bso@odoo.com>

addons/website/models/website.py

@@ -1186,6 +1186,7 @@ class Website(models.Model):
             domain = []
         domain += self.get_current_website().website_domain()
         pages = self.env['website.page'].sudo().search(domain, order=order, limit=limit)
+        pages = pages._get_most_specific_pages()
         return pages
 
     def search_pages(self, needle=None, limit=None):