/usr/include/x86_64-linux-gnu/ is in libsolv0-dev 0.6.5-1.
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/usr/include/x86_64-linux-gnu/solv/
/usr/include/x86_64-linux-gnu/solv/bitmap.h
/usr/include/x86_64-linux-gnu/solv/chksum.h
/usr/include/x86_64-linux-gnu/solv/dataiterator.h
/usr/include/x86_64-linux-gnu/solv/dirpool.h
/usr/include/x86_64-linux-gnu/solv/evr.h
/usr/include/x86_64-linux-gnu/solv/hash.h
/usr/include/x86_64-linux-gnu/solv/knownid.h
/usr/include/x86_64-linux-gnu/solv/policy.h
/usr/include/x86_64-linux-gnu/solv/pool.h
/usr/include/x86_64-linux-gnu/solv/poolarch.h
/usr/include/x86_64-linux-gnu/solv/poolid.h
/usr/include/x86_64-linux-gnu/solv/pooltypes.h
/usr/include/x86_64-linux-gnu/solv/poolvendor.h
/usr/include/x86_64-linux-gnu/solv/problems.h
/usr/include/x86_64-linux-gnu/solv/queue.h
/usr/include/x86_64-linux-gnu/solv/repo.h
/usr/include/x86_64-linux-gnu/solv/repodata.h
/usr/include/x86_64-linux-gnu/solv/rules.h
/usr/include/x86_64-linux-gnu/solv/selection.h
/usr/include/x86_64-linux-gnu/solv/solvable.h
/usr/include/x86_64-linux-gnu/solv/solver.h
/usr/include/x86_64-linux-gnu/solv/solverdebug.h
/usr/include/x86_64-linux-gnu/solv/solvversion.h
/usr/include/x86_64-linux-gnu/solv/strpool.h
/usr/include/x86_64-linux-gnu/solv/testcase.h
/usr/include/x86_64-linux-gnu/solv/transaction.h
/usr/include/x86_64-linux-gnu/solv/util.h