FetchContent: Avoid including ExternalProject more than once

Even with the call to include_guard() at the top of the ExternalProject
module, including it still shows up in profiling runs. It only needs to be
included once when we include FetchContent, not every time we do a
population.

Relates: #21703
29 jobs for !5782 with fetchcontent-performance-round2 in 11 minutes and 33 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
manual #5518762
linux docker cmake build manual
build:centos6-x86_64
passed #5518759
linux docker cmake build
build:debian10-iwyu

00:05:51

manual #5518763
linux docker cmake build linux-3.17 manual
build:fedora33-ninja
passed #5518761
linux docker cmake build linux-3.17
build:fedora33-sphinx

00:02:11

passed #5518760
linux docker cmake build linux-3.17
build:fedora33-tidy

00:11:33

manual #5518765
cmake macos shell nonconcurrent xcode-12.3 manual
build:macos-makefiles
manual #5518764
cmake macos shell nonconcurrent xcode-12.3 manual
build:macos-ninja
manual #5518766
windows cmake shell vs2019 nonconcurrent msvc-19.28 manual
build:windows-vs2019-x64-ninja
 
  Test
created #5518767
linux docker cmake x11 linux-3.17
test:fedora33-ninja
created #5518769
cmake macos shell nonconcurrent xcode-12.3
test:macos-makefiles
created #5518768
cmake macos shell nonconcurrent xcode-12.3
test:macos-ninja
created #5518770
windows cmake shell vs2019 nonconcurrent msvc-19.28
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5518773
linux docker cmake cuda-rt
test:cuda10.2-nvidia
created #5518771
linux docker cmake build linux-3.17
test:debian10-ninja
created #5518772
linux docker cmake build linux-3.17
test:fedora33-makefiles
created #5518774
linux docker cmake build linux-3.17
test:fedora33-ninja-multi
created #5518775
cmake macos shell concurrent xcode-12.3
test:macos-xcode
created #5518776
windows cmake shell vs2019 concurrent msvc-19.28
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager