Add debug message.

debexpo/plugins/debtags.py

@@ -108,6 +108,8 @@ class DebtagsPlugin(BasePlugin):
         autotag.load_sources(self.binary_packages)
 
         tags = autotag.get_tags()
+        if len(tags) == 0:
+            log.debug('No tag found by heuristics.')
         for pkg in tags:
             for tag in tags[pkg]:
                 self._new_result(Debtag, tag = tag, source = 'autotag',