reference: introduce const for "localhost"

Localhost is treated special when parsing references, and always considered
to be a domain, despite not having a "." nor a ":port". Adding a const for
this, to allow documenting this special case (making it more visible).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

reference/normalize.go

@@ -125,7 +125,7 @@ func ParseDockerRef(ref string) (Named, error) {
 // needs to be already validated before.
 func splitDockerDomain(name string) (domain, remainder string) {
 	i := strings.IndexRune(name, '/')
-	if i == -1 || (!strings.ContainsAny(name[:i], ".:") && name[:i] != "localhost" && strings.ToLower(name[:i]) == name[:i]) {
+	if i == -1 || (!strings.ContainsAny(name[:i], ".:") && name[:i] != localhost && strings.ToLower(name[:i]) == name[:i]) {
 		domain, remainder = defaultDomain, name
 	} else {
 		domain, remainder = name[:i], name[i+1:]

reference/regexp.go

@@ -19,6 +19,10 @@ const (
 	// supported names.
 	separator = `(?:[._]|__|[-]*)`
 
+	// localhost is treated as a special value for domain-name. Any other
+	// domain-name without a "." or a ":port" are considered a path component.
+	localhost = `localhost`
+
 	// domainNameComponent restricts the registry domain component of a
 	// repository name to start with a component as defined by DomainRegexp.
 	domainNameComponent = `(?:[a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])`