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:

4f3d1abb (ExternalProject: Refactor pre-configure steps to support
no-target uses, 2021-02-05)

17e5516e (FetchContent: Invoke steps directly and avoid a separate
sub-build, 2021-01-29)

bd876f38 (FetchContent: Restore patch command support,
2021-02-18)

404cddb7 (ExternalProject: Fix misuse of IS_NEWER_THAN in
timestamp checks, 2021-02-21)

b0da6712 (FetchContent: Don't update timestamps if files don't
change, 2021-02-18)

Fixes: #21892