Fix issues caught by piuparts

Both -client and -export were missing dependencies on
desktop-file-utils, which is needed for the `update-desktop-database`
call in the postinst. -export also needed shared-mime-info.

As far as I can tell, the -dev and libtool-bin dependencies in -export
are unnecessary; we aren't compiling any C/C++ code at runtime that
should need them.

Finally, make the postrm commands optional per
<https://www.debian.org/doc/debian-policy/ch-relationships.html#binary-dependencies-depends-recommends-suggests-enhances-pre-depends>,
which states that "There is no guarantee that package dependencies will
be available when postrm is run".

debian/control

@@ -9,12 +9,13 @@ X-Python3-Version: >= 3.5
 
 Package: securedrop-client
 Architecture: all
-Depends: ${python3:Depends},${misc:Depends}, python3-pyqt5, python3-pyqt5.qtsvg, apparmor-utils
+Depends: ${python3:Depends},${misc:Depends}, python3-pyqt5, python3-pyqt5.qtsvg, apparmor-utils, desktop-file-utils
 Description: securedrop client for qubes workstation
 
 Package: securedrop-export
 Architecture: all
-Depends: ${python3:Depends}, ${misc:Depends}, cryptsetup, cups, printer-driver-brlaser, printer-driver-hpcups, system-config-printer, xpp, libcups2-dev, python3-dev, libtool-bin, unoconv, gnome-disk-utility
+Depends: ${python3:Depends}, ${misc:Depends}, cryptsetup, cups, printer-driver-brlaser, printer-driver-hpcups, system-config-printer, xpp, libcups2, unoconv, gnome-disk-utility,
+ desktop-file-utils, shared-mime-info
 Description: Submission export scripts for SecureDrop Workstation
  This package provides scripts used by the SecureDrop Qubes Workstation to
  export submissions from the client to external storage, via the sd-export

debian/securedrop-export.postrm

@@ -22,8 +22,14 @@ set -e
 case "$1" in
     purge|remove|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
 
-    update-desktop-database /usr/share/applications
-    update-mime-database /usr/share/mime
+    # There is no guarantees dependencies are available in postrm, so
+    # conditionally issue these updates.
+    if [[ -x "/usr/bin/update-desktop-database" ]]; then
+        update-desktop-database /usr/share/applications
+    fi
+    if [[ -x "/usr/bin/update-mime-database" ]]; then
+        update-mime-database /usr/share/mime
+    fi
     ;;
 
     *)