Redo the parsing of packages from primary.xml to use SAX; previously it
was using pulldom. The motivation for the change is to reduce memory usage.
When parsing a larger yum repo such as that contained within
rhel-8-for-ppc64le-appstream-kickstart__8_DOT_4, the observed memory
usage from repo-autoindex command was:
- pulldom: ~700MB
- SAX: ~85MB
This does not affect the output of the indexing process, and is covered
by existing tests.
Ultimately, all errors are propagated in some way, but it's important to
differentiate between "the content was invalid" vs "failed to fetch the
content".