portable: remove drop-in configs even if the main unit file does not exist

portable: remove drop-in configs even if the main unit file does not exist

When we run `portablectl detach --enable --runtime`, then it triggers
`DisableUnitFilesWithFlags` DBus method and the main unit file is
removed, but its drop-ins are not. Hence, portable_detach() failed to
list existing portable units.

This makes the loop for listing portable units also accept drop-in
directories. So, all remaining drop-in directories are correctly
removed.

Before:
```
testsuite-29.sh[600]: + portablectl detach --now --runtime --enable /tmp/rootdir minimal-app0
portablectl[1391]: (Matching unit files with prefixes 'minimal-app0'.)
portablectl[1391]: Queued /org/freedesktop/systemd1/job/1812 to call StopUnit on portable service minimal-app0-foo.service.
portablectl[1391]: Removed "/run/systemd/system.attached/minimal-app0-foo.service".
portablectl[1391]: Queued /org/freedesktop/systemd1/job/1813 to call StopUnit on portable service minimal-app0.service.
portablectl[1391]: Removed "/run/systemd/system.attached/minimal-app0.service".
portablectl[1391]: Got result done/Success for job minimal-app0-foo.service
portablectl[1391]: Got result done/Success for job minimal-app0.service
portablectl[1391]: DetachImage failed: No unit files associated with '/tmp/rootdir' found attached to the system. Image not attached?
```

After:
```
testsuite-29.sh[508]: + portablectl detach --now --runtime --enable /tmp/rootdir minimal-app0
portablectl[1076]: (Matching unit files with prefixes 'minimal-app0'.)
portablectl[1076]: Queued /org/freedesktop/systemd1/job/1946 to call StopUnit on portable service minimal-app0-foo.service.
portablectl[1076]: Removed "/run/systemd/system.attached/minimal-app0-foo.service".
portablectl[1076]: Queued /org/freedesktop/systemd1/job/1947 to call StopUnit on portable service minimal-app0.service.
portablectl[1076]: Removed "/run/systemd/system.attached/minimal-app0.service".
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0.service.d/10-profile.conf.
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0.service.d/20-portable.conf.
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0.service.d.
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0-foo.service.d/10-profile.conf.
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0-foo.service.d/20-portable.conf.
portablectl[1076]: Removed /run/systemd/system.attached/minimal-app0-foo.service.d.
portablectl[1076]: Removed /run/portables/rootdir.
portablectl[1076]: Removed /run/systemd/system.attached.
```