cooker: Further optimise pyinotify

We currently add crazy numbers of watches on files. The per user limit is 8192 by default and on a system handling multiple builds, this can be an issue.

We don't need to watch all files individually, we can watch the directory containing the file instead. This gives better resource utilisation and better performance further reverting some of the performance regression seen with the introduction of pyinotify.

index 9086f92..554dfbd 100644
@@ -182,7 +182,7 @@ class BBCooker:
         if not watcher:
             watcher = self.watcher
         for i in deps:
-            f = i[0]
+            f = os.path.dirname(i[0])
             if f in watcher.bbseen:
                 continue
             watcher.bbseen.append(f)