Revert ExternalProject and FetchContent refactoring

Refactoring of the ExternalProject and FetchContent modules moved
the commands into CMake scripts. This broke custom commands that
used shell redirection or special build tool variables of the form
$(MakeVar). Undo the sequence of commits that performed this
refactoring and follow-up fixes associated with it.

The following commits are reverted by this change:

4f3d1abbb4 (ExternalProject: Refactor pre-configure steps to support
no-target uses, 2021-02-05)

17e5516e60 (FetchContent: Invoke steps directly and avoid a separate
sub-build, 2021-01-29)

bd876f3849 (FetchContent: Restore patch command support,
2021-02-18)

404cddb7bb (ExternalProject: Fix misuse of IS_NEWER_THAN in
timestamp checks, 2021-02-21)

b0da671243 (FetchContent: Don't update timestamps if files don't
change, 2021-02-18)

Fixes: #21892

Modules/ExternalProject/gitclone.cmake.in

@@ -1,93 +0,0 @@
-# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
-# file Copyright.txt or https://cmake.org/licensing for details.
-
-cmake_minimum_required(VERSION 3.5)
-
-set(quiet "@quiet@")
-set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
-include(${script_dir}/captured_process_setup.cmake)
-
-if(EXISTS "@gitclone_stampfile@" AND EXISTS "@gitclone_infofile@" AND
-   "@gitclone_stampfile@" IS_NEWER_THAN "@gitclone_infofile@")
-  if(NOT quiet)
-    message(STATUS
-      "Avoiding repeated git clone, stamp file is up to date: "
-      "'@gitclone_stampfile@'"
-    )
-  endif()
-  return()
-endif()
-
-execute_process(
-  COMMAND ${CMAKE_COMMAND} -E rm -rf "@source_dir@"
-  RESULT_VARIABLE error_code
-  ${capture_output}
-)
-_ep_command_check_result(
-  error_code "Failed to remove directory: '@source_dir@'"
-)
-
-# try the clone 3 times in case there is an odd git clone issue
-set(error_code 1)
-set(number_of_tries 0)
-while(error_code AND number_of_tries LESS 3)
-  # If you are seeing the following call hang and you have QUIET enabled, try
-  # turning QUIET off to show any output immediately. The command may be
-  # blocking while waiting for user input (e.g. a password to a SSH key).
-  execute_process(
-    COMMAND "@git_EXECUTABLE@" @git_options@
-            clone @git_clone_options@ "@git_repository@" "@src_name@"
-    WORKING_DIRECTORY "@work_dir@"
-    RESULT_VARIABLE error_code
-    ${capture_output}
-  )
-  if(NOT "${out_var}" STREQUAL "")
-    string(APPEND accumulated_output "${out_var}\n")
-  endif()
-  math(EXPR number_of_tries "${number_of_tries} + 1")
-endwhile()
-if(number_of_tries GREATER 1)
-  set(msg "Had to git clone more than once: ${number_of_tries} times.")
-  if(quiet)
-    string(APPEND accumulated_output "${msg}\n")
-  else()
-    message(STATUS "${msg}")
-  endif()
-endif()
-_ep_command_check_result(
-  error_code "Failed to clone repository: '@git_repository@'"
-)
-
-execute_process(
-  COMMAND "@git_EXECUTABLE@" @git_options@
-          checkout "@git_tag@" @git_checkout_explicit--@
-  WORKING_DIRECTORY "@work_dir@/@src_name@"
-  RESULT_VARIABLE error_code
-  ${capture_output}
-)
-_ep_command_check_result(error_code "Failed to checkout tag: '@git_tag@'")
-
-set(init_submodules @init_submodules@)
-if(init_submodules)
-  execute_process(
-    COMMAND "@git_EXECUTABLE@" @git_options@
-            submodule update @git_submodules_recurse@ --init @git_submodules@
-    WORKING_DIRECTORY "@work_dir@/@src_name@"
-    RESULT_VARIABLE error_code
-    ${capture_output}
-  )
-  _ep_command_check_result(
-    error_code "Failed to update submodules in: '@work_dir@/@src_name@'"
-  )
-endif()
-
-# Complete success, update the script-last-run stamp file:
-#
-execute_process(
-  COMMAND ${CMAKE_COMMAND} -E copy "@gitclone_infofile@" "@gitclone_stampfile@"
-  RESULT_VARIABLE error_code
-  ${capture_output}
-)
-_ep_command_check_result(
-  error_code "Failed to copy script-last-run stamp file: '@gitclone_stampfile@'"
-)