Move HelpRequest classes to help package. (#21861)

And delete arg_splitter.py, as those were 
the last remaining bits of code in that file.

src/python/pants/goal/help.py

@@ -16,20 +16,24 @@
 from pants.engine.unions import UnionMembership
 from pants.goal.builtin_goal import BuiltinGoal
 from pants.help.help_info_extracter import HelpInfoExtracter
-from pants.help.help_printer import HelpPrinter
-from pants.init.engine_initializer import GraphSession
-from pants.option.arg_splitter import (
-    NO_GOAL_NAME,
-    UNKNOWN_GOAL_NAME,
+from pants.help.help_printer import (
     AllHelp,
+    HelpPrinter,
     HelpRequest,
     NoGoalHelp,
     ThingHelp,
     UnknownGoalHelp,
     VersionHelp,
 )
+from pants.init.engine_initializer import GraphSession
 from pants.option.options import Options
 
+# These are the names for the built in goals to print help message when there is no goal, or any
+# unknown goals respectively. They begin with underlines to exclude them from the list of goals in
+# the goal help output.
+NO_GOAL_NAME = "__no_goal"
+UNKNOWN_GOAL_NAME = "__unknown_goal"
+
 
 class HelpBuiltinGoalBase(BuiltinGoal):
     def run(

src/python/pants/help/help_printer.py

@@ -5,6 +5,8 @@
 import json
 import re
 import textwrap
+from abc import ABC
+from dataclasses import dataclass
 from itertools import cycle
 from typing import Callable, Dict, Iterable, List, Literal, Optional, Set, Tuple, cast
 
@@ -13,19 +15,43 @@
 from pants.help.help_info_extracter import AllHelpInfo, HelpJSONEncoder
 from pants.help.help_tools import ToolHelpInfo
 from pants.help.maybe_color import MaybeColor
-from pants.option.arg_splitter import (
-    AllHelp,
-    HelpRequest,
-    NoGoalHelp,
-    ThingHelp,
-    UnknownGoalHelp,
-    VersionHelp,
-)
 from pants.option.scope import GLOBAL_SCOPE
 from pants.util.docutil import bin_name, terminal_width
 from pants.util.strutil import first_paragraph, hard_wrap, pluralize, softwrap
 
 
+class HelpRequest(ABC):
+    """Represents an implicit or explicit request for help by the user."""
+
+
+@dataclass(frozen=True)
+class ThingHelp(HelpRequest):
+    """The user requested help on one or more things: e.g., an options scope or a target type."""
+
+    advanced: bool = False
+    things: tuple[str, ...] = ()
+    likely_specs: tuple[str, ...] = ()
+
+
+class VersionHelp(HelpRequest):
+    """The user asked for the version of this instance of pants."""
+
+
+class AllHelp(HelpRequest):
+    """The user requested a dump of all help info."""
+
+
+@dataclass(frozen=True)
+class UnknownGoalHelp(HelpRequest):
+    """The user specified an unknown goal (or task)."""
+
+    unknown_goals: tuple[str, ...]
+
+
+class NoGoalHelp(HelpRequest):
+    """The user specified no goals."""
+
+
 class HelpPrinter(MaybeColor):
     """Prints general and goal-related help to the console."""