Debug symbols missed from Debian packages

dbg packages for Debian 9 and Debian 8 contain debug binaries and miss debug symbols.

How to repeat

1. wget https://www.percona.com/downloads/Percona-XtraDB-Cluster-56/Percona-XtraDB-Cluster-5.6.41-28.28/binary/debian/stretch/x86_64/percona-xtradb-cluster-5.6-dbg_5.6.41-28.28-1.stretch_amd64.deb
2. # dpkg-deb -x percona-xtradb-cluster-5.6-dbg_5.6.41-28.28-1.stretch_amd64.deb tmp
3. root@2f3167ab02d9:/opt/data1# find tmp -name *debug
   tmp/usr/lib/debug
   tmp/usr/lib/debug/usr/sbin/mysqld-debug
   root@2f3167ab02d9:/opt/data1# ls -a tmp/usr/
   lib/ share/
   root@2f3167ab02d9:/opt/data1# ls -a tmp/usr/
   . .. lib share
   root@2f3167ab02d9:/opt/data1# ls -a tmp/usr/lib/debug/
   . .. usr
   root@2f3167ab02d9:/opt/data1# ls -a tmp/usr/lib/debug/usr/
   . .. bin lib sbin share
   root@2f3167ab02d9:/opt/data1# ls -a tmp/usr/lib/debug/usr/lib/
   . .. mysql percona-xtradb-cluster-testsuite x86_64-linux-gnu
   root@2f3167ab02d9:/opt/data1#

Expected behavior:

Have debug symbols