Allow to skip more options to update-walker in simple cases. Now just an URL can work if everything is set up in default way

pkgs/build-support/upstream-updater/update-walker.sh

···
 own_dir="$(cd "$(dirname "$0")"; pwd)"
 
 CURRENT_URL=
+NEED_TO_CHOOSE_URL=1
 
 url () {
   CURRENT_URL="$1"
···
 
 link () {
   CURRENT_URL="$(matching_links "$1" | position_choice "$2" "$3")"
+  unset NEED_TO_CHOOSE_URL
   echo "Linked by: $*"
   echo "URL: $CURRENT_URL" >&2
 }
 
 version_link () {
   CURRENT_URL="$(matching_links "$1" | version_sort | position_choice "$2" "$3")"
+  unset NEED_TO_CHOOSE_URL
   echo "Linked version by: $*"
   echo "URL: $CURRENT_URL" >&2
 }
···
   [ -z "$CURRENT_TARGET" ] && target default.nix
 }
 
+ensure_name () {
+  [ -z "$CURRENT_NAME" ] && name "$(basename "$CONFIG_DIR")"
+  echo "Resulting name: $CURRENT_NAME"
+}
+
+ensure_choice () {
+  [ -n "NEED_TO_CHOOSE_URL" ] && {
+    version_link '[.]tar[.]([^./])+$'
+    unset NEED_TO_CHOOSE_URL
+  }
+  [ -z "$CURRENT_URL" ] && {
+    echo "Error: empty CURRENT_URL"
+    echo "Error: empty CURRENT_URL" >&2
+    exit 1
+  }
+}
+
 hash () {
   CURRENT_HASH="$(nix-prefetch-url "$CURRENT_URL")"
 }
···
 
 process_config () {
   CONFIG_DIR="$(directory_of "$1")"
-  source "$CONFIG_DIR/$(basename "$1")"
   BEGIN_EXPRESSION='# Generated upstream information';
+  source "$CONFIG_DIR/$(basename "$1")"
+  ensure_name
   retrieve_version
+  ensure_choice
   ensure_version
   ensure_target
   update_found && do_overwrite "$CURRENT_TARGET"