The library finder needs to have capitals in its name so that FIND_PACKAGE
will load the correct finder and actually detect that libzstd is available.

Without this change the CMake would just always silently skip ZSTD since
it would never find it.

Simplify Debian autopkgtest RocksDB part and make it more verbose so that
future regressions like this are easier to debug.

Also remove QUIET from the RocksDB FIND_PACKAGE call so that it is easier
to read in build logs what libraries were detected. Also add missing
underscores to error messages.