dgit: Option parsing: Fix undefined $suite in some import-dsc.

dgit: Option parsing: Fix undefined $suite in some import-dsc.

* Set $isuite to a dummy value.  In import-dsc we aren't operating on
  a suite.  We have to set it to something (because the config likes
  to look at the suite) and this avoids picking up information
  relating to the default suite.

* Set $idistro (if -d not supplied) to the distro from (or implied by)
  the .dsc.  This is needed so that we do not use any config from the
  default distro.  In particular, the default commands (like what
  `git' command to use to access the dgit git server) need to be found
  via the dsc's distro's cmd-git setting, and this is done by
  parseopts_late_defaults() in pushing().  So we must set $idistro
  before then.

* Move notpushing earlier, so that we call it before we call
  resolve_dsc_field_commit().  (It doesn't matter there for $idistro,
  because it sets that itself, but the git command does.)

Closes:#851213.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>