Move version printing to version package

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/main.go

@@ -3,7 +3,6 @@ package main
 import (
 	"flag"
 	"fmt"
-	"io"
 	"net/http"
 	_ "net/http/pprof"
 	"os"
@@ -34,7 +33,7 @@ func main() {
 	flag.Parse()
 
 	if showVersion {
-		printVersion(os.Stdout)
+		version.PrintVersion()
 		return
 	}
 
@@ -59,10 +58,6 @@ func usage() {
 	flag.PrintDefaults()
 }
 
-func printVersion(w io.Writer) {
-	fmt.Fprintln(w, os.Args[0], version.Package, version.Version)
-}
-
 func fatalf(format string, args ...interface{}) {
 	fmt.Fprintf(os.Stderr, format+"\n", args...)
 	usage()

version/print.go

@@ -0,0 +1,26 @@
+package version
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+// FprintVersion outputs the version string to the writer, in the following
+// format, followed by a newline:
+//
+// 	<cmd> <project> <version>
+//
+// For example, a binary "registry" built from github.com/docker/distribution
+// with version "v2.0" would print the following:
+//
+// 	registry github.com/docker/distribution v2.0
+//
+func FprintVersion(w io.Writer) {
+	fmt.Fprintln(w, os.Args[0], Package, Version)
+}
+
+// PrintVersion outputs the version information, from Fprint, to stdout.
+func PrintVersion() {
+	FprintVersion(os.Stdout)
+}