Archives are stored under pkg:composer/{vendor}/{package}, which purl lowercases, so case in a package rule doesn't matter. A key without a vendor could never match and is refused. Packages that git-pkgs wrote as "packagist" fall under the composer rules too.
The mirror can't fetch composer packages today (its resolver has no composer support, and #415 would file them under packagist), so there's no mirror test.
Refs #306
