[api] need URLs to binary changes, build output files

While attempting to convert the ubuntu-security publication LP HTML-parser to the LP API, I found several things that are not exposed to the API that we need:

- source_package_publishing_history needs links to all of the source input files (e.g. .dsc, .diff.gz, .tar.gz, etc URLs)
  - we currently scrape the "Package files" section for the .dsc, .diff.gz, .tar.gz shown in +archive/ppa/+sourcepub/NNN/+listing-archive-extra

- binary_package_publishing_history needs links to all of the build output files (e.g. .changes, .deb, .udeb, etc URLs)
  - we currently scrape the "Package files" section for the .debs shown in +archive/ppa/+sourcepub/NNN/+listing-archive-extra
  - we currently scrape the "Changes file" section for .changes files shown in +archive/ppa/+build/NNN

It would also be nice to have an easier-to-find answer to the "is this an arch-all package?" question. It seems that from the API, this is answered by:
  arch_all = False
  for src in archive.getPublishedSources(source_name=pkg_name, status='Published'):
    for bin in src.getPublishedBinaries():
      if bin.distro_arch_series.is_nominated_arch_indep:
        arch_all = True

This is rather "expensive", I would hope for replacing the last three lines with:
    if src.is_arch_indep:
      arch_all = True