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/mobi/writer8/exth.py

@@ -2,6 +2,7 @@ import re
 from struct import pack
 from io import BytesIO
 
+from ebook_converter.ebooks.oeb import base
 from ebook_converter.constants_old import iswindows, isosx
 from ebook_converter.ebooks.mobi.utils import (utf8_text, to_base)
 from ebook_converter.utils.localization import lang_as_iso639_1
@@ -95,9 +96,8 @@ def build_exth(metadata, prefer_author_sort=False, is_periodical=False,
 
     # Write UUID as ASIN
     uuid = None
-    from ebook_converter.ebooks.oeb.base import OPF
     for x in metadata['identifier']:
-        if (x.get(OPF('scheme'), None).lower() == 'uuid' or
+        if (x.get(base.tag('opf', 'scheme'), None).lower() == 'uuid' or
                 str(x).startswith('urn:uuid:')):
             uuid = str(x).split(':')[-1]
             break