Use the real constants module.

This is progressing refactor of the calibre code to make it more
readable, and transform it to something more coherent.

In this patch, there are changes regarding imports for some modules,
instead of polluting namespace of each module with some other modules
symbols, which often were imported from other modules. Yuck.

ebook_converter/ebooks/oeb/transforms/page_margin.py

@@ -1,7 +1,8 @@
 import numbers
 from collections import Counter
 
-from ebook_converter.ebooks.oeb.base import barename, XPath
+from ebook_converter.ebooks.oeb import parse_utils
+from ebook_converter.ebooks.oeb.base import XPath
 
 
 __license__ = 'GPL v3'
@@ -142,7 +143,7 @@ class RemoveFakeMargins(object):
 
             for p in paras(body):
                 level = level_of(p, body)
-                level = '%s_%d'%(barename(p.tag), level)
+                level = '%s_%d' % (parse_utils.barename(p.tag), level)
                 if level not in self.levels:
                     self.levels[level] = []
                 self.levels[level].append(p)