Change backtracking when packages conflict too much #667

Job	Run time
linux-s390x (s390x-unknown-linux-gnu, s390x)	14m 29s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 5s
linux (x86_64-unknown-linux-gnu)	8m 0s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 44s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 40s
linux (i686-unknown-linux-gnu)	11m 16s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	10m 46s
macos-aarch64	13m 47s
macos-x86_64	12m 52s
linux-arm (arm-unknown-linux-musleabihf, arm)	9m 28s
musllinux (x86_64-unknown-linux-musl)	10m 9s
musllinux (i686-unknown-linux-musl)	10m 26s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 35s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	10m 55s
sdist	9m 26s
windows (x86_64-pc-windows-msvc, x64)	15m 30s
windows (i686-pc-windows-msvc, x86)	15m 43s
	3h 16m 51s

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