Rule's menu parent is set as active link for pages that are already in the menu.

Problem/Motivation

A rule's menu parent is set as the active link for pages that have a real menu link, but also match the rule.

Steps to reproduce

1. Create a menu as following:

• Main Navigation
• About Us
• Our History

2. Add a Main Navigation menu block to the block layout:
- Initial visibility level: 1
- Number of levels to display: 4

3. Create a menu_position rule with Main Navigation > About Us as the menu parent,
and Path as the criteria, with the path rule set to `/about-us/*`

4. Create a node that provides a Menu link and set its parent to `Our History`

5. View the node.

Expectation: The menu should display with `Our History` set as parent, since the page has an actual menu entry.
From the module's README: "if a menu item for the current page's path is already included in the specified menu, the rule is skipped."

Actual Behavior: The menu is displayed with `About Us` set as parent.

Confirmation: If you disable the menu_position rule and reload the page, the menu is displayed with `Our History` set as parent.

Proposed resolution

Drupal\menu_position\Menu\MenuPositionActiveTrail::getActiveLink() should check for the existence of an active link before applying the rule.