The dh_missing helper checks that all the files from the tmp directory
have an installation rule. Otherwise:
- With compat level < 12: it silently ignores the files
- With compat level 12: it warns about the presence of these files
- With compat level > 12: it errors out in the presence of these files
This diff improves our build wrt dh_missing by avoiding putting files
to the tmp directory which are not installed (namely the
bitcoinconsensus library and the seeder), and enforce dh_missing to
raise a warning if some files remain. It doesn't change the current
behavior since we are using compat level 12 but will prevent future
builds from failing if this level is bumped.