wxr import: post_parent is handled incorrectly in some circumstances

[tellyworth]

The patch in #5357 handles parent/child pages incorrectly in some cases, when there are multiple levels of nesting and the posts are included in the least optimal order in the import file. The Level 1 / Level 2 / Level 3 pages in this export file reproduce the problem:

 http://svn.automattic.com/wordpress-tests/wp-testdata/export/asdftestblog1.2007-12-11.xml

The enclosed patch replaces the two pass processing from #5357 with a simpler method. Posts are imported in a single pass, then the post_parent of child pages are updated in backfill_parents() later, once the local post ID's of all parents are known. I also removed a little cruft and split the import() function in two for easier testing.

This passes the updated unit tests here:

 http://svn.automattic.com/wordpress-tests/wp-testcase/test_import_wp.php

Revised patch fixes a problem with categories

(In [6374]) Correctly handle post_parent during import. Props tellyworth. fixes #5456

Fixed on trunk.