search: accept entire:// mirror remotes

entire search derived owner/repo via a GitHub-only parser that required the remote host to be github.com, so it rejected Entire mirror remotes (entire://host/gh/owner/repo) with "remote is not a GitHub repository".

Delegate ParseGitHubRemote to the canonical gitremote.ParseURL, which understands the entire:// scheme, and enforce the GitHub policy via CanonicalHost() (gh -> github.com). This also fixes the same limitation in the two dispatch callers that share the helper, and removes the duplicated URL parsing.