locking packages not working as expected when dependencies need to be locked too

Describe the bug

Locking an installed package version that is not available (in an enabled repo) does not work as intended with distro-sync, when there is a dependency that is also not available.

Reproduction steps

1. install package with a version not available in a repo (for example a locally built package), example tdnf
2. Lock it with

mkdir /etc/tdnf/locks.d
echo tdnf > /etc/tdnf/locks.d/tdnf.conf

3. try tdnf distro-sync

Actual result:

root [ ~ ]# mkdir /etc/tdnf/locks.d
root [ ~ ]# echo tdnf > /etc/tdnf/locks.d/tdnf.conf
root [ ~ ]# tdnf distro-sync
1. package tdnf-3.5.2-1.ph4.x86_64 requires tdnf-cli-libs = 3.5.2-1.ph4, but none of the providers can be installed
Found 1 problem(s) while resolving
Error(1301) : Solv general runtime error

Expected behavior

All packages except tdnf will be sync'ed to the available repos. No downgrade of tdnf.

Additional context

To workaround, lock the installed dependency as well:

echo tdnf-cli-libs > /etc/tdnf/locks.d/tdnf.conf

# cat /etc/tdnf/locks.d/tdnf.conf
tdnf-cli-libs
tdnf

# tdnf distro-sync
Nothing to do.
root [ ~ ]# 

Start by reproducing the issue with the documented /etc/tdnf/locks.d/tdnf.conf entries and tdnf distro-sync, comparing behavior when only tdnf or both packages are locked. Trace the distro-sync dependency-resolution path and verify that an unavailable locked package does not prevent other packages from syncing or trigger its downgrade.