Add support for modular build structure. #426

Job	Run time
windows_msvc_14_3 (msvc-14.3)	22m 14s
macos (clang, 14)	5m 0s
windows_msvc_14_2 (msvc-14.2)	22m 2s
posix-cmake-install (ubuntu-22.04)	1m 25s
ubuntu (gcc-14, 14)	6m 42s
macos (clang, 17)	5m 34s
posix-cmake-install (ubuntu-24.04)	1m 30s
ubuntu (gcc-14, 17)	6m 55s
windows-cmake-install (windows-2019)	5m 40s
macos (clang, 20)	5m 53s
posix-cmake-install (macos-13)	3m 1s
ubuntu (gcc-14, 20)	7m 44s
windows-cmake-install (windows-2022)	4m 7s
posix-cmake-install (macos-14)	1m 21s
ubuntu (gcc-14, 23)	9m 26s
posix-cmake-install (macos-15)	1m 34s
windows-cmake-subdir (windows-2019)	2m 51s
ubuntu (clang-19, 14)	7m 43s
windows-cmake-subdir (windows-2022)	3m 36s
ubuntu (clang-19, 17)	9m 3s
posix-cmake-subdir (ubuntu-22.04)	1m 17s
posix-cmake-subdir (ubuntu-24.04)	1m 22s
ubuntu (clang-19, 20)	9m 11s
posix-cmake-subdir (macos-13)	2m 30s
ubuntu (clang-19, 23)	10m 56s
posix-cmake-subdir (macos-14)	1m 38s
posix-cmake-subdir (macos-15)	1m 36s
	2h 41m 51s

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