mixins: moved debug logs to 'filter_file'. Renamed shadowed variable name.

Following comments from Todd:

- the call to tty.debug has been moved deeper, to log the filtering of each file
- the shadowing on the name "kwargs" is avoided

lib/spack/llnl/util/filesystem.py

@@ -119,9 +119,15 @@ def groupid_to_group(x):
         regex = re.escape(regex)
 
     for filename in filenames:
+
+        msg = 'FILTER FILE: {0} [replacing "{1}"]'
+        tty.debug(msg.format(filename, regex))
+
         backup_filename = filename + "~"
 
         if ignore_absent and not os.path.exists(filename):
+            msg = 'FILTER FILE: file "{0}" not found. Skipping to next file.'
+            tty.debug(msg.format(filename))
             continue
 
         # Create backup file. Don't overwrite an existing backup