aws-ecr: repositoryName of imported repository may be incorrect

[eladb]

Repro:

const repo = Repository.import(this, 'MyRepo', { 
  repositoryArn: 'arn:aws:ecr:us-east-1:585695036304:repository/foo/bar/foo/fooo'
});

Now, we expect repo.repositoryName to resolve to foo/bar/foo/fooo but it will resolve to foo.

This is because ImportedRepository uses CFN's split/select to extract the first component after "/".

[eladb]

If the user provides a concrete (non-token) string, we can just use synth-time string manipulation to extract the full repository name. If repositoryArn is a token, we will have to require that repositoryName will also be passed.