Add Arch Linux ecosystem and fix distro-namespace purl lookups

[andrew]

Adds support for indexing Arch Linux packages from both the official repos and the AUR.

One Ecosystem::Arch class with two registries, branching on registry.metadata['kind'] (official / aur):

• Official: paginated archlinux.org/packages/search/json/ for the full list (~16k), /feeds/packages/ RSS for recently updated, pacman -S install command
• AUR: aur.archlinux.org/packages.gz for the full list (~111k), /rpc/v5/info for metadata, /rss/modified + /rss/ via Nokogiri for recently updated (SimpleRSS chokes on the single-quoted XML declaration), yay -S install command
• Dependencies parsed from depends/makedepends/checkdepends/optdepends with version constraint splitting (pacman>6.1 → name pacman, requirement >6.1) and optdepend description stripping
• Maintainers from both sources
• purl type alpm, namespace arch, version [epoch:]pkgver-pkgrel, qualifiers arch and upstream — matches what syft emits for archlinux:base (trivy doesn't catalog ALPM packages at all)

Also fixes purl lookups for distro-namespace types. SBOM scanners emit pkg:alpm/arch/jq, pkg:apk/alpine/jq, pkg:deb/debian/curl where the namespace is the distro, not part of the package name. The lookup code was joining namespace into the name (arch/jq) and only resolving one ecosystem per purl type, so apk and deb lookups returned nothing. Changes:

• Ecosystem::Base.name_from_purl centralises name construction (replaces three copies of the join logic)
• purl_namespace_in_name? hook, set false on Arch, Deb, Alpine, Adelie, Postmarketos
• purl_type_to_ecosystems (plural) so apk resolves to alpine/adelie/postmarketos and deb to deb/debian/ubuntu; all four registry-lookup callsites updated
• self.purl_type added to alpine/adelie/postmarketos (apk) and debian/ubuntu (deb)

To enable in production:

Registry.find_or_create_by(name: 'archlinux.org', url: 'https://archlinux.org', ecosystem: 'arch', github: 'archlinux', default: true, metadata: {'kind' => 'official'}).sync_all_packages_async
Registry.find_or_create_by(name: 'aur.archlinux.org', url: 'https://aur.archlinux.org', ecosystem: 'arch', github: 'archlinux', default: false, metadata: {'kind' => 'aur'}).sync_all_packages_async

Closes #1613