EPUB writer:  Don't add sourceURL to absolute URIs!

Closes #1669.

If there are further issues, please open a new, targeted issue on the
tracker.  Some notes on the further issues you gestured at:

Data URIs are indeed dereferenced, but why is this a problem?
(The function being used to fetch from URLs is used for many different
formats.  Preserving data URIs would make sense in EPUBs, but not
for e.g. PDF output.  And by dereferencing we can get a smaller,
more efficient EPUB, with the data stored as bytes in a file rather
than encoded in textual representation.)

"absolute uris are not recognized" -- I assume that is the problem
just fixed.  If not, please open a new issue.

"relative uris are resolved (wrongly) like file paths" -- can you
give an example?

`<base>` tag is ignored.  Yes. I didn't know about the base tag.  Could
you open a new issue just for this?