EPUB-related: fix some errors and warnings

 * **epub3-pub-utils:**
   * added more log messages, both for easier debugging and more insight into what is happening when running the step
   * replaced a p:count/p:choose/re-pipe assertion with px:assert
   * putting wrapper elements in the `xmlns:d="http://www.daisy.org/ns/pipeline/data"` namespace
   * added a template in create-metadata.merge.xsl and create-package-doc.fileset-resolve.xsl that matches the wrapper elements explicitly. fixes warning:
     `err:SXXP0005:The source document is in no namespace, but the template rules all expect elements in a namespace`
 * **epub3-nav-utils:**
   * in nav-to-ncx.xsl, select root element instead of document node. fixes fatal error:
   `err:XTTE0790:Required item type of third argument of pf:i18n-translate() is element(); supplied value has item type document-node(element(Q{http://www.daisy.org/ns/pipeline/data}i18n))`
   * when no language was determined, don't add `@lang` and `@xml:lang` to the navigation document (px:epub3-nav-aggregate)
   * read `html:meta[@name='dc:identifier']/@content` instead of `html:meta[@name='dc:identifier']` when determining the dtb:uid (nav-to-ncx.xsl)
 * **mediaoverlay-utils**:
   * added templates to match the internal wrapper element `di:smil-map` explicitly. fixes warning:
     `err:SXXP0005:The source document is in namespace http://www.daisy.org/ns/pipeline/tmp, but none of the template rules match elements in this namespace`