refactor: move external procfile manipulation into triggers

This also cleans up the internal APIs a bit.

plugins/ps/procfile-get-command

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/ps/functions"
+
+trigger-ps-procfile-get-command() {
+  declare desc="ps procfile-get-command plugin trigger"
+  declare trigger="procfile-get-command"
+  declare desc="extract command from an app procfile"
+  declare APP="$1" PROCESS_TYPE="$2" PORT="$3" "$PROCFILE_PATH"
+
+  if [[ -z "$PROCFILE_PATH" ]]; then
+    PROCFILE_PATH="$DOKKU_LIB_ROOT/data/ps/$APP/Procfile.extracted"
+  fi
+
+  fn-procfile-get-command "$PROCFILE_PATH" "$PROCESS_TYPE" "$PORT"
+}
+
+trigger-ps-procfile-get-command "$@"