cargo: Fix feature resolution #16771

Job	Run time
Arch Linux	19m 47s
CUDA (on Arch)	7m 34s
Fedora	19m 48s
Gentoo	22m 32s
OpenSUSE	16m 11s
Ubuntu Bionic	15m 3s
Arch / PyPy	19m 52s
Ubuntu Rolling (gcc, g++)	20m 3s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	14m 10s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 50s
Ubuntu Rolling (clang, clang++)	22m 13s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 20s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 50s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 9s
	3h 53m 22s

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