Split V3Partition into logically separate pieces (#4958)

V3Partition used to contain 2 conceptually separate set of algorithms

- The MTask partitioning/coarsening algorithm used by V3Order. This has
  been moved to V3OrderParallel.cpp

- The lowering of AstExecGraph into per thread functions by packing
  tasks into threads and creating additional code
  (V3Partition::finalize). This has been moved to the new
  V3ExecGraph.cpp

This patch is just code movement/rename with minimal fixes required to
do so.

src/V3ExecGraph.h

@@ -0,0 +1,33 @@
+// -*- mode: C++; c-file-style: "cc-mode" -*-
+//*************************************************************************
+// DESCRIPTION: Verilator: AstExecGraph code construction
+//
+// Code available from: https://verilator.org
+//
+//*************************************************************************
+//
+// Copyright 2003-2024 by Wilson Snyder. This program is free software; you
+// can redistribute it and/or modify it under the terms of either the GNU
+// Lesser General Public License Version 3 or the Perl Artistic License
+// Version 2.0.
+// SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+//
+//*************************************************************************
+
+#ifndef VERILATOR_V3EXECGRAPH_H_
+#define VERILATOR_V3EXECGRAPH_H_
+
+#include "config_build.h"
+#include "verilatedos.h"
+
+#include "V3ThreadSafety.h"
+
+class AstNetlist;
+
+namespace V3ExecGraph {
+void implement(AstNetlist*) VL_MT_DISABLED;
+
+void selfTest() VL_MT_DISABLED;
+}  //namespace V3ExecGraph
+
+#endif  // Guard