As of libsolv 0.7.7 at least, libsolv assumes that the rpmdb has a file named Packages in /var/lib/rpm, which is not true with other backends. Upstream has a workaround (see ticket https://github.com/openSUSE/libsolv/issues/354), this is mainly for tracking purposes on Fedora.
For the record, with these commits libsolv 0.7.7 appears functional with alternative backends for testing purposes at least: https://github.com/openSUSE/libsolv/commit/94df5d6bf2eec3fc555550307342e250e2b842c6 https://github.com/openSUSE/libsolv/commit/ea037722b0f2a1afc8416e08b36fd498a8bb14c4
FEDORA-2019-0735fed56c has been submitted as an update to Fedora 31. https://bodhi.fedoraproject.org/updates/FEDORA-2019-0735fed56c
FEDORA-2019-b9a002c8c9 has been submitted as an update to Fedora 30. https://bodhi.fedoraproject.org/updates/FEDORA-2019-b9a002c8c9
libsolv-0.7.8-1.fc30 has been pushed to the Fedora 30 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2019-b9a002c8c9
libsolv-0.7.8-1.fc31 has been pushed to the Fedora 31 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2019-0735fed56c
libsolv-0.7.8-1.fc30 has been pushed to the Fedora 30 stable repository. If problems still persist, please make note of it in this bug report.
libsolv-0.7.8-1.fc31 has been pushed to the Fedora 31 stable repository. If problems still persist, please make note of it in this bug report.