cldr-plurals: Rewrite XML handling using libxml2

cldr-plurals: Rewrite XML handling using libxml2

* src/cldr-plurals.c: Include <libxml/tree.h> and
<libxml/parser.h>, instead of "libexpat-compat.h".
(extract_locale, extracted_rules, parser, logical_file_name)
(struct element_state, stack, stack_size, stack_depth)
(ensure_stack_size, start_element_handler, end_element_handler)
(character_data_handler): Remove.
(extract_rules): Rename from extract_rule and rewrite using DOM.
(main): Use extract_rules.
* src/Makefile.am (cldr_plurals_CFLAGS): Add $(INCXML).