Import map merging

[CertainLach]

My implementation of multiple import maps in deno (https://github.com/denoland/deno/pull/30754) iterates over import maps to find matches. It is, however, unable to detect if ImportMap::resolve has returned an entry, that should be tried from next import map.

This change allows to only return remapped paths.

This PR also implements import map merging as described by spec: https://html.spec.whatwg.org/multipage/webappapis.html#merge-existing-and-new-import-maps I can split those changes if that would be better for review purposes

Ref: https://github.com/denoland/deno/pull/30754#issuecomment-3305359983