Define order of package fields in merged Translations file.

Under python 3.5.3, the dictionary order is undefined. Ensure that
the Package is the first line in its stanza. APT doesn't seem to mind, but it
makes the file much harder to read.

amprolla_merge_translations.py

@@ -64,9 +64,9 @@ def write_translations(translations, filename):
     bz2f = bz2_open(filename, 'w')
 
     for pkg, fields in sorted(translations.items()):
-        for key in fields.keys():
-            if key != 'Package' and not key.startswith('Description-'):
-                continue
+        ordered_keys = ['Package']
+        ordered_keys.extend([k for k in fields.keys() if k.startswith('Description-')])
+        for key in ordered_keys:
             line = "%s: %s\n" % (key, fields[key])
             bz2f.write(line.encode('utf-8'))
         bz2f.write(b'\n')