Extract logic for the URI variable identifying the exercise to download

There is very likely a better abstraction hiding close by.
This is a minimal, cohesive change, which I think is fairly uncontroversial.

If we do find the better abstraction, it's likely that we'll be able to move this one
to it wholesale.

I've left a bit of duplication in place for now, rather than make a big
guess about the abstraction.

cmd/download.go

@@ -54,44 +54,40 @@ func runDownload(cfg config.Config, flags *pflag.FlagSet, args []string) error {
 		return err
 	}
 
-	uuid, err := flags.GetString("uuid")
+	track, err := flags.GetString("track")
 	if err != nil {
 		return err
 	}
-	slug, err := flags.GetString("exercise")
+
+	team, err := flags.GetString("team")
 	if err != nil {
 		return err
 	}
-	if uuid != "" && slug != "" || uuid == slug {
-		return errors.New("need an --exercise name or a solution --uuid")
-	}
 
-	track, err := flags.GetString("track")
+	identifier, err := downloadIdentifier(flags)
 	if err != nil {
 		return err
 	}
+	url := fmt.Sprintf("%s/solutions/%s", usrCfg.GetString("apibaseurl"), identifier)
 
-	team, err := flags.GetString("team")
+	client, err := api.NewClient(usrCfg.GetString("token"), usrCfg.GetString("apibaseurl"))
 	if err != nil {
 		return err
 	}
 
-	param := "latest"
-	if uuid != "" {
-		param = uuid
+	req, err := client.NewRequest("GET", url, nil)
+	if err != nil {
+		return err
 	}
-	url := fmt.Sprintf("%s/solutions/%s", usrCfg.GetString("apibaseurl"), param)
 
-	client, err := api.NewClient(usrCfg.GetString("token"), usrCfg.GetString("apibaseurl"))
+	uuid, err := flags.GetString("uuid")
 	if err != nil {
 		return err
 	}
-
-	req, err := client.NewRequest("GET", url, nil)
+	slug, err := flags.GetString("exercise")
 	if err != nil {
 		return err
 	}
-
 	if uuid == "" {
 		q := req.URL.Query()
 		q.Add("exercise_id", slug)
@@ -252,6 +248,27 @@ func (dp downloadPayload) metadata() workspace.ExerciseMetadata {
 	}
 }
 
+// downloadIdentifier is the variable for the URI to initiate an exercise download.
+func downloadIdentifier(flags *pflag.FlagSet) (string, error) {
+	uuid, err := flags.GetString("uuid")
+	if err != nil {
+		return "", err
+	}
+	slug, err := flags.GetString("exercise")
+	if err != nil {
+		return "", err
+	}
+	if uuid != "" && slug != "" || uuid == slug {
+		return "", errors.New("need an --exercise name or a solution --uuid")
+	}
+
+	identifier := "latest"
+	if uuid != "" {
+		identifier = uuid
+	}
+	return identifier, nil
+}
+
 func setupDownloadFlags(flags *pflag.FlagSet) {
 	flags.StringP("uuid", "u", "", "the solution UUID")
 	flags.StringP("track", "t", "", "the track ID")