Investigate replacing custom XML parsing with libxml2

[TingPing]

Currently in builder-utils.c there is semi-custom parsing built upon GMarkupParser.

Since we already depend on libxml2 indirectly through libappstream we might as well use a more robust parser.

[hfiguiere]

Actually it already directly depends on libxml.

https://github.com/flatpak/flatpak-builder/blob/main/src/meson.build#L58C1-L58C47