Excluding other extensions when an extension is present

Right now we have some extensions that don't work well together. For example when `` and `` works together, the ladder component can affect what the previous component does.
It would be useful to have a feature that only allow one component on a page among a list of mutual exclusive extensions.

@ampproject/wg-caching

Start by reading the amp-skimlink and amp-link-rewrite extension definitions and how the ladder component interacts with them. Trace where extensions are selected or initialized on a page; done means a configured mutual-exclusion list prevents more than one listed extension from affecting the same page without breaking other extensions.

Written by the indexing model from the issue text.