pkgconfig: Handle malformatted string dependencies #17134

Job	Run time
Arch Linux	19m 58s
CUDA (on Arch)	7m 10s
Fedora	17m 59s
Gentoo	22m 13s
OpenSUSE	16m 1s
Ubuntu Bionic	14m 43s
Arch / PyPy	19m 37s
Ubuntu Rolling (gcc, g++)	19m 52s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 41s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 1s
Ubuntu Rolling (clang, clang++)	22m 42s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 9s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 48s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 22s
	3h 49m 16s

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