PACKAGES.rds comes from the same host as the tarball, so checking against it
doesn't justify a decoder for R serialization. The duplicate archived
versions are tracked in git-pkgs/registries#119.
proxy mirror pkg:cran/<name>@<version> now fetches the source tarball
from upstream.cran and caches it under the key the CRAN handler reads,
("cran", name, version, name_version.tar.gz), which serves both
/src/contrib/{file} and /src/contrib/Archive/{name}/{file}.
A current version comes from src/contrib and is checked against the
MD5sum in src/contrib/PACKAGES.gz, read once per five minutes. Other
versions come from src/contrib/Archive/{name}/, checked against the
MD5sum in that directory's PACKAGES.rds, an R-serialized character
matrix read by a small decoder. Without a PACKAGES.rds the archived
tarball is fetched unchecked, as the handler fetches it.
Listing versions dropped duplicates: the CRAN client lists each archived
version twice.
