Make Jpegli standalone, remove most part of libjxl code #17

Job	Run time
Ubuntu Build release	4m 57s
Ubuntu Build debug	1m 53s
Ubuntu Build asan	2m 54s
Ubuntu Build msan	4m 1s
Ubuntu Build tsan	3m 11s
Ubuntu Build scalar	1m 43s
Ubuntu Build lowprecision	4m 53s
Ubuntu Build coverage	1m 14s
Ubuntu Build release-nojpeg	2m 11s
Ubuntu Build release-lcms2	1m 59s
Ubuntu Build release-system-lcms2	1m 59s
Ubuntu Build release:minimal	2m 8s
Ubuntu Build release:gcc8	1m 35s
Ubuntu Build release:clang-7	1m 58s
MacOS Build release:osx	1m 26s
	38m 2s

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