refactor: move external procfile manipulation into triggers

This also cleans up the internal APIs a bit.

plugins/ps/procfile-extract

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/ps/functions"
+
+trigger-ps-procfile-extract() {
+  declare desc="ps procfile-extract plugin trigger"
+  declare trigger="procfile-extract"
+  declare desc="extract procfile from a specified docker image"
+  declare APP="$1" IMAGE="$2" FORCE_EXTRACT="$3" PROCFILE_PATH="$4"
+
+  if [[ -z "$PROCFILE_PATH" ]]; then
+    PROCFILE_PATH="$DOKKU_LIB_ROOT/data/ps/$APP/Procfile.extracted"
+  fi
+
+  if [[ "$FORCE_EXTRACT" == "true" ]]; then
+    rm "$PROCFILE_PATH"
+  fi
+
+  fn-procfile-extract "$APP" "$IMAGE" "$PROCFILE_PATH"
+}
+
+trigger-ps-procfile-extract "$@"