Replace references to "not installable" packages with "non-installable"

This reduces the number of ways in which we refer to uninstallable packages
to two.  Those should probably be unified further to be consistent across
all parts of the code, but in the meantime this ensures that methods are
internally consistent in their terminology.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -251,7 +251,7 @@ class Britney:
         # if this or the population of self.binaries below takes a very
         # long time, try increasing SIZEOFHASHMAP in lib/dpkg.c and rebuilding
         if not self.options.nuninst_cache:
-            self.__log("Building the list of not installable packages for the full archive", type="I")
+            self.__log("Building the list of non-installable packages for the full archive", type="I")
             self.sources = {'testing': self.read_sources(self.options.testing)}
             nuninst = {}
             for arch in self.options.architectures:
@@ -266,7 +266,7 @@ class Britney:
             if not self.options.print_uninst:
                 self.write_nuninst(nuninst)
         else:
-            self.__log("Not building the list of not installable packages, as requested", type="I")
+            self.__log("Not building the list of non-installable packages, as requested", type="I")
 
         # if running in --print-uninst mode, quit here
         if self.options.print_uninst: