proxy mirror and mirror jobs now take pkg:julia PURLs with the uuid
qualifier. The tree hash of the version comes from the General registry,
after checking the uuid is the one General lists for the name, and the
tarball is fetched from upstream.julia at /package/{uuid}/{tree-hash} and
cached under the name and tree hash the Julia handler uses.
The handler names a package from the registry's Registry.toml, which it
loads from the upstream. Without the upstream it fell back to caching and
looking up by uuid, so a package cached under its name, mirrored or
fetched before a restart, was not served offline. It now looks the
tarball up by its tree hash in that case.
Julia versions are listed from General whatever upstream.julia is, as
packages are identified by uuid and tree hash on every Pkg server serving
General.
