fix: re-structure update check

Signed-off-by: jolheiser <john.olheiser@gmail.com>

main.go

@@ -56,10 +56,18 @@ func main() {
 	}
 	uri := fs.Arg(0)
 
-	if *updateFlag && strings.Index(uri, "/") == -1 {
-		if pkg, ok := m.Packages[uri]; ok {
-			uri = pkg.Repo
+	if strings.Index(uri, "/") != -1 {
+		if !*updateFlag {
+			fmt.Println("provided arg must be a URI, unless using --update")
+			return
 		}
+
+		pkg, ok := m.Packages[uri]
+		if !ok {
+			fmt.Println("arg was not a URI and didn't match an installed package")
+			return
+		}
+		uri = pkg.Repo
 	}
 
 	f, err = forge.NewGitea(uri)