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/polish/container.py

@@ -14,7 +14,9 @@ from itertools import count
 import urllib.parse
 
 from css_parser import getUrls, replaceUrls
+from lxml import etree
 
+from ebook_converter import constants as const
 from ebook_converter import CurrentDir, walk
 from ebook_converter.constants_old import iswindows
 from ebook_converter.customize.ui import plugin_for_input_format, plugin_for_output_format
@@ -34,7 +36,7 @@ from ebook_converter.ebooks.mobi import MobiError
 from ebook_converter.ebooks.mobi.reader.headers import MetadataHeader
 from ebook_converter.ebooks.mobi.tweak import set_cover
 from ebook_converter.ebooks.oeb.base import (
-    DC11_NS, OEB_DOCS, OEB_STYLES, OPF, OPF2_NS, Manifest, itercsslinks, iterlinks,
+    OEB_DOCS, OEB_STYLES, Manifest, itercsslinks, iterlinks,
     rewrite_links, serialize, urlquote, urlunquote
 )
 from ebook_converter.ebooks.oeb.parse_utils import NotHTML, parse_html
@@ -47,13 +49,11 @@ from ebook_converter.ptempfile import PersistentTemporaryDirectory, PersistentTe
 from ebook_converter.utils.filenames import hardlink_file, nlinks_file
 from ebook_converter.utils.ipc.simple_worker import WorkerError, fork_job
 from ebook_converter.utils.logging import default_log
-from ebook_converter.utils.xml_parse import safe_xml_fromstring
 from ebook_converter.utils.zipfile import ZipFile
 
 exists, join, relpath = os.path.exists, os.path.join, os.path.relpath
 
 OEB_FONTS = {guess_type('a.ttf'), guess_type('b.otf'), guess_type('a.woff'), 'application/x-font-ttf', 'application/x-font-otf', 'application/font-sfnt'}
-OPF_NAMESPACES = {'opf':OPF2_NS, 'dc':DC11_NS}
 null = object()
 
 
@@ -195,7 +195,7 @@ class ContainerBase(object):  # {{{
         data, self.used_encoding = xml_to_unicode(
             data, strip_encoding_pats=True, assume_utf8=True, resolve_entities=True)
         data = unicodedata.normalize('NFC', data)
-        return safe_xml_fromstring(data)
+        return etree.fromstring(data)
 
     def parse_xhtml(self, data, fname='<string>', force_html5_parse=False):
         if self.tweak_mode:
@@ -324,7 +324,7 @@ class Container(ContainerBase):  # {{{
             item_id = 'id' + '%d'%c
         manifest = self.opf_xpath('//opf:manifest')[0]
         href = self.name_to_href(name, self.opf_name)
-        item = manifest.makeelement(OPF('item'),
+        item = manifest.makeelement(const.OPF_ITEM,
                                     id=item_id, href=href)
         item.set('media-type', self.mime_map[name])
         self.insert_into_xml(manifest, item)
@@ -380,7 +380,7 @@ class Container(ContainerBase):  # {{{
         if mt in OEB_DOCS:
             manifest = self.opf_xpath('//opf:manifest')[0]
             spine = self.opf_xpath('//opf:spine')[0]
-            si = manifest.makeelement(OPF('itemref'), idref=item_id)
+            si = manifest.makeelement(const.OPF_ITEMREF, idref=item_id)
             self.insert_into_xml(spine, si, index=spine_index)
         return name
 
@@ -533,7 +533,7 @@ class Container(ContainerBase):  # {{{
 
     def opf_xpath(self, expr):
         ' Convenience method to evaluate an XPath expression on the OPF file, has the opf: and dc: namespace prefixes pre-defined. '
-        return self.opf.xpath(expr, namespaces=OPF_NAMESPACES)
+        return self.opf.xpath(expr, namespaces=const.OPF_NAMESPACES)
 
     def has_name(self, name):
         ''' Return True iff a file with the same canonical name as that specified exists. Unlike :meth:`exists` this method is always case-sensitive. '''
@@ -813,7 +813,8 @@ class Container(ContainerBase):  # {{{
         spine = self.opf_xpath('//opf:spine')[0]
         spine.text = tail
         for name, linear in spine_items:
-            i = spine.makeelement('{%s}itemref' % OPF_NAMESPACES['opf'], nsmap={'opf':OPF_NAMESPACES['opf']})
+            i = spine.makeelement(const.OPF_ITEMREF,
+                                  nsmap={'opf': const.OPF2_NS})
             i.tail = tail
             i.set('idref', imap[name])
             spine.append(i)
@@ -944,7 +945,7 @@ class Container(ContainerBase):  # {{{
             item_id = id_prefix + '%d'%c
 
         manifest = self.opf_xpath('//opf:manifest')[0]
-        item = manifest.makeelement(OPF('item'),
+        item = manifest.makeelement(const.OPF_ITEM,
                                     id=item_id, href=href)
         item.set('media-type', media_type)
         self.insert_into_xml(manifest, item)
@@ -993,7 +994,7 @@ class Container(ContainerBase):  # {{{
             self.format_opf()
         data = serialize(data, self.mime_map[name], pretty_print=name in
                          self.pretty_print)
-        if name == self.opf_name and root.nsmap.get(None) == OPF2_NS:
+        if name == self.opf_name and root.nsmap.get(None) == const.OPF2_NS:
             # Needed as I can't get lxml to output opf:role and
             # not output <opf:metadata> as well
             data = re.sub(br'(<[/]{0,1})opf:', r'\1', data)
@@ -1172,7 +1173,7 @@ class EpubContainer(Container):
         container_path = join(self.root, 'META-INF', 'container.xml')
         if not exists(container_path):
             raise InvalidEpub('No META-INF/container.xml in epub')
-        container = safe_xml_fromstring(open(container_path, 'rb').read())
+        container = etree.fromstring(open(container_path, 'rb').read())
         opf_files = container.xpath((
             r'child::ocf:rootfiles/ocf:rootfile'
             '[@media-type="%s" and @full-path]'%guess_type('a.opf')