Using preserve_facet_query_args can cause double query parameters in pager links on views

Problem/Motivation

When using preserve_facet_query_args the exposed filter form gets the facet items injected in search_api_form_views_exposed_form_alter. When the pager manager then adds query parameters to each pager link it uses \Drupal\Core\Pager\PagerManager::getUpdatedParameters where the query parameters are in the form f->0->facet_item while the form has the items as f[0]->facet_item. This means facet items are added twice.

This means the pager causes a "spider trap" where search engines just keep finding pages to index. This destroys the SEO ranking of a site basically.

Steps to reproduce

* Create a search_api view with an exposed filter (e.g. for full-text search), make sure the preserve_facet_query_args setting is set
* Create a facet for the view, e.g. for a taxonomy term.
* Go to the page with the view and facet and use a facet
* Inspect the pager links.

Proposed resolution

* I tried updating search_api_form_views_exposed_form_alter to add the items as `$form["{$filter_key}"][$key] = [` instead of `$form["{$filter_key}[$key]"] = [`, which was a very stupid idea and didn't work. The other option involves updating core, which would be unmanageable.

At the moment I fixed it with the patch in attachment, but it feels like a less than ideal solution.

Remaining tasks