refactor: remove action runners from public dune

Action runners in this dune are a rather old version that it is very
much out of date with the state of the actual feature and has many bugs
and limitations that need a lot of upstreaming to fix. In its current
state, it does not provide any value to users of public dune.

If we ever want to upstream this feature, it will be easier to just do
it from scratch anyway instead of resolving the conflicts against some
old version. This is due to the fact that the action runner API with the
engine is rather small and well defined.

Once we introduce a better abstraction for action execution (as we are
indeed planning), it will be probably be even simpler to upstream action
runners.

All in all, we gain nothing on either team by keeping this dead weight
around.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: b913f829-def9-4d41-90d6-07da8060063b -->

bin/common.ml

@@ -552,14 +552,6 @@ let cache_debug_flags_term : Cache_debug_flags.t Term.t =
   value initial
 ;;
 
-module Action_runner = struct
-  type t =
-    | No
-    | Yes of
-        (Dune_lang.Dep_conf.t Dune_rpc_impl.Server.t
-         -> (Dune_engine.Action_exec.input -> Dune_engine.Action_runner.t option) Staged.t)
-end
-
 module Builder = struct
   type t =
     { debug_dep_path : bool
@@ -601,14 +593,12 @@ module Builder = struct
     ; stats_trace_extended : bool
     ; allow_builds : bool
     ; default_root_is_cwd : bool
-    ; action_runner : Action_runner.t
     ; log_file : Dune_util.Log.File.t
     }
 
   let set_root t root = { t with root = Some root }
   let forbid_builds t = { t with allow_builds = false; no_print_directory = true }
   let set_default_root_is_cwd t x = { t with default_root_is_cwd = x }
-  let set_action_runner t x = { t with action_runner = x }
   let set_log_file t x = { t with log_file = x }
   let disable_log_file t = { t with log_file = No_log_file }
   let set_promote t v = { t with promote = Some v }
@@ -1028,7 +1018,6 @@ module Builder = struct
     ; stats_trace_extended
     ; allow_builds = true
     ; default_root_is_cwd = false
-    ; action_runner = No
     ; log_file = Default
     }
   ;;
@@ -1159,16 +1148,14 @@ let build (builder : Builder.t) =
              | Yes Passive -> Some 1.0
              | _ -> None
            in
-           let action_runner = Dune_engine.Action_runner.Rpc_server.create () in
            Dune_rpc_impl.Server.create
              ~lock_timeout
              ~registry
              ~root:root.dir
              ~handle:Dune_rules_rpc.register
              ~watch_mode_config:builder.watch
              ~parse_build:Dune_rules_rpc.parse_build
-             stats
-             action_runner))
+             stats))
     else `Forbid_builds
   in
   if builder.print_metrics then Dune_metrics.enable ();
@@ -1225,16 +1212,7 @@ let init (builder : Builder.t) =
         "Shared cache location: %s"
         (Path.to_string Dune_cache_storage.Layout.root_dir)
     ];
-  let action_runner =
-    match builder.action_runner with
-    | No -> None
-    | Yes f ->
-      (match rpc c with
-       | `Forbid_builds -> Code_error.raise "action runners require building" []
-       | `Allow server -> Some (Staged.unstage @@ f server))
-  in
   Dune_rules.Main.init
-    ?action_runner
     ~stats:c.stats
     ~sandboxing_preference:config.sandboxing_preference
     ~cache_config

bin/common.mli

@@ -24,22 +24,13 @@ val watch : t -> Dune_rpc_impl.Watch_mode_config.t
 val file_watcher : t -> Dune_engine.Scheduler.Run.file_watcher
 val prefix_target : t -> string -> string
 
-module Action_runner : sig
-  type t =
-    | No
-    | Yes of
-        (Dune_lang.Dep_conf.t Dune_rpc_impl.Server.t
-         -> (Dune_engine.Action_exec.input -> Dune_engine.Action_runner.t option) Staged.t)
-end
-
 (** [Builder] describes how to initialize Dune. *)
 module Builder : sig
   type t
 
   val set_root : t -> string -> t
   val forbid_builds : t -> t
   val set_default_root_is_cwd : t -> bool -> t
-  val set_action_runner : t -> Action_runner.t -> t
   val set_log_file : t -> Dune_util.Log.File.t -> t
   val disable_log_file : t -> t
   val set_promote : t -> Dune_engine.Clflags.Promote.t -> t