Return a clear error message when the URL is provided for a repository, organization or enterprise, when the name should be provided

[timrogers]

We have regularly seen customers have issues with the CLI where they use a URL for something (e.g. a repository, organization or enterprise) rather than its name.

Relevant examples include the --github-org, --github-target-org, --source-repo and --github-target-enterprise arguments.

We should validate these arguments and return a clear warning if a URL is provided rather than a name.

We could alternatively consider fixing this in the GraphQL API - but it may prove to be non-trivial because of the GraphQL framework.