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/jacket.py

@@ -4,9 +4,10 @@ from string import Formatter
 import pkg_resources
 import urllib.parse
 
+from ebook_converter import constants as const
 from ebook_converter import guess_type, strftime
 from ebook_converter.constants_old import iswindows
-from ebook_converter.ebooks.oeb.base import XPath, XHTML_NS, XHTML, xml2text, urlnormalize
+from ebook_converter.ebooks.oeb.base import XPath, xml2text, urlnormalize
 from ebook_converter.library.comments import comments_to_html, markdown
 from ebook_converter.utils.date import is_date_undefined, as_local_time
 from ebook_converter.ebooks.chardet import strip_encoding_declarations
@@ -303,7 +304,7 @@ def render_jacket(mi, output_profile,
                 'tags_label': 'Tags',
                 'title': title,
                 'title_str': title_str,
-                'xmlns': XHTML_NS}
+                'xmlns': const.XHTML_NS}
 
         for key in mi.custom_field_keys():
             m = mi.get_user_metadata(key, False) or {}
@@ -370,7 +371,7 @@ def render_jacket(mi, output_profile,
         # We cannot use data-calibre-rescale 100 on the body tag as that will just
         # give the body tag a font size of 1em, which is useless.
         for body in root.xpath('//*[local-name()="body"]'):
-            fw = body.makeelement(XHTML('div'))
+            fw = body.makeelement(const.XHTML_DIV)
             fw.set('data-calibre-rescale', '100')
             for child in body:
                 fw.append(child)
@@ -387,9 +388,9 @@ def linearize_jacket(oeb):
     for x in oeb.spine[:4]:
         if XPath(JACKET_XPATH)(x.data):
             for e in XPath('//h:table|//h:tr|//h:th')(x.data):
-                e.tag = XHTML('div')
+                e.tag = const.XHTML_DIV
             for e in XPath('//h:td')(x.data):
-                e.tag = XHTML('span')
+                e.tag = const.XHTML_SPAN
             break