Add support for modular build structure. #224

Job	Run time
posix (STD=14 JOB=test/x3, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-dev jq cc...	4m 7s
posix (STD=14 JOB=test/x3, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 14, test/x3, gcc-11)	3m 23s
posix (STD=11 JOB=test/qi, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-dev jq cc...	8m 13s
posix (STD=11 JOB=test/karma, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-dev jq...	7m 28s
posix (STD=11 JOB=test/lex, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-dev jq c...	6m 6s
posix (STD=11 JOB=test/support, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-dev ...	2m 20s
posix (STD=11 JOB=repository/test, boost, clang-14 libc++-14-dev libc++abi-14-dev libunwind-14-d...	2m 27s
posix (STD=11 JOB=test/qi, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 11, test/qi, gcc-11)	7m 25s
posix (STD=11 JOB=test/karma, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 11, test/karma, gcc...	6m 48s
posix (STD=11 JOB=test/lex, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 11, test/lex, gcc-11)	5m 9s
posix (STD=11 JOB=test/support, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 11, test/support,...	1m 58s
posix (STD=11 JOB=repository/test, boost, g++-11 jq ccache, ubuntu-22.04, gcc-11, 11, repository...	2m 13s
	57m 37s

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