Fix: Make --registry-map compatible with namespaced images

[mlallaouret]

With --registry-map=="index.docker.io=europe-west1-docker.pkg.dev/my-registry/docker-hub", when we try to pull the image eclipse-temurin:21.0.2_13-jre, as expected we get:

INFO Retrieving image manifest eclipse-temurin:21.0.2_13-jre 
INFO Retrieving image my-registry/docker-hub/eclipse-temurin:21.0.2_13-jre from mapped registry europe-west1-docker.pkg.dev  

However with the same --registry-map option, when we try to pull the image caddy/caddy:2.7.6-alpine, we get:

INFO Retrieving image manifest caddy/caddy:2.7.6-alpine 
INFO Retrieving image caddy/caddy:2.7.6-alpine from mapped registry europe-west1-docker.pkg.dev 
WARN Failed to retrieve image caddy/caddy:2.7.6-alpine from remapped registry europe-west1-docker.pkg.dev: unable to complete operation after 0 attempts, last error: GET https://europe-west1-docker.pkg.dev/v2/token?scope=repository%3Acaddy%2Fcaddy%3Apull&service=: DENIED: Permission "artifactregistry.repositories.downloadArtifacts" denied on resource "projects/caddy/locations/europe-west1/repositories/caddy" (or it may not exist). Will try with the next registry, or fallback to the original registry. 

Description

• For each registry mapping, represent it by a new instance of Repository and create a new Reference containing it.
• Improve registry mapping parser
• Add more unit tests to cover more use cases

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Examples of user facing changes:
- Fix: Make `--registry-map` compatible with namespaced images