Using the URL alias UI to change aliases doesn't do necessary invalidations: path aliases don't have cache tags

Problem/Motivation

Quoting @effulgentsia in #2335661-23: Outbound path & route processors must specify cacheability metadata:

See also #2417793-46: Allow entity: URIs to be entered in link fields, where changes to URL aliases don't result in a cache clear of rendered entities containing formatted links. Maybe that needs its own issue, but seems highly related to this, so just noting it here for now.

Reply by @Wim Leers in #2335661-98: Outbound path & route processors must specify cacheability metadata:

This is exactly what I feared was the case. Note that when changing the URL alias of a node, the node updates a MenuLinkContent entity. Which means everything is updated correctly. It is only when updating URL aliases via /admin/config/search/path, that no invalidation happens. But, to be honest, that's really a problem independent of this issue: URL aliases don't track changes and hence also not invalidations in any way. This issue is about fixing the more general problem, that sits a level higher: to make it possible for those outbound path/route processors that do have cacheability metadata, to be able to associate that cacheability with generated URLs.
So we should fix URL aliases in a separate issue.

This is that separate issue.

Proposed resolution

As written in #12:

When rendering

for all route params that are used to generate the link, associate the cache tags of those that implement CacheableDependencyInterface with the rendered link

When saving an alias in the URL alias UI

find the route name + params associated with the source path (=== "run routing")

get the set of cache tags from the route params

if empty set of cache tags: invalidate 'rendered' cache tag

if non-empty set of cache tags: invalidate those cache tags

Remaining tasks

Do it.

User interface changes

None.

API changes

None.