mmdebstrap: Show APT's dependency trace when in debug mode

A dependency trace is a powerful tool to debug issues related to
APT's selection of packages, especially in custom mode.

Thus it makes sense to ask APT to output a dependency trace when
`mmdebstrap` is run with the `--debug` flag.

mmdebstrap

@@ -2085,6 +2085,9 @@ sub run_download() {
                     'apt-get',
                     '--yes',
                     '-oApt::Get::Download-Only=true',
+                    ($verbosity_level >= 3) ? '-oDebug::pkgProblemResolver=true' : (),
+                    ($verbosity_level >= 3) ? '-oDebug::pkgDepCache::Marker=1' : (),
+                    ($verbosity_level >= 3) ? '-oDebug::pkgDepCache::AutoInstall=1' : (),
                     $options->{dryrun} ? '-oAPT::Get::Simulate=true' : (),
                     'install'
                 ],