common: allow files that don't exist to be URLs [GH-683]

CHANGELOG.md

@@ -11,6 +11,7 @@ BUG FIXES:
 
 * core: Don't change background color on CLI anymore, making things look
   a tad nicer in some terminals.
+* core: multiple ISO URLs works properly in all builders. [GH-683]
 * builder/amazon/chroot: Block when obtaining file lock to allow
   parallel builds. [GH-689]
 * builder/vmware: Cleanup of VMX keys works properly so cd-rom won't

common/config.go

@@ -105,10 +105,9 @@ func DownloadableURL(original string) (string, error) {
 			url.Path = strings.Replace(url.Path, `\`, `/`, -1)
 		}
 
-		if _, err := os.Stat(url.Path); err != nil {
-			return "", err
-		}
-
+		// Only do the filepath transformations if the file appears
+		// to actually exist.
+		if _, err := os.Stat(url.Path); err == nil {
 			url.Path, err = filepath.Abs(url.Path)
 			if err != nil {
 				return "", err
@@ -121,6 +120,7 @@ func DownloadableURL(original string) (string, error) {
 
 			url.Path = filepath.Clean(url.Path)
 		}
+	}
 
 	// Make sure it is lowercased
 	url.Scheme = strings.ToLower(url.Scheme)