env2mfile: Add unit test coverage for dpkg -> Meson, and fix some obvious errors #16653

Job	Run time
Arch / PyPy	20m 17s
Arch Linux	21m 21s
Ubuntu Rolling (gcc, g++)	20m 18s
CUDA (on Arch)	7m 57s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 52s
Fedora	20m 21s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 35s
Gentoo	21m 41s
Ubuntu Rolling (clang, clang++)	22m 11s
OpenSUSE	16m 32s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 35s
Ubuntu Bionic	15m 7s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 4s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 44s
	3h 57m 35s

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