Use the Pkg.Registry API

[danielmatz]

As of Julia 1.7, Pkg.Registry provides some of the capability that had
been implemented in this package. This commit makes this package's
reachable_registries a simple wrapper around
Pkg.Registry.reachable_registries and removes the RegistryInstance
and PkgEntry types in favor of the types of the same name in
Pkg.Registry.

The largest change is that this package's version of
RegistryInstance stored a dictionary mapping package names to
PkgEntrys, but Pkg.Registry's version has a dictionary mapping
UUIDs to PkgEntrys.

A benefit of using the Pkg.Registry API is that it already supports
both uncompressed registries as well as registries stored in
compressed tarballs. Thus, this fixes #37.

[danielmatz]

We now use @static to keep the original implementation in place, but then use the new Pkg.Registry capabilities for Julia v1.7 and later. This seems to work, though we'll have to be very careful going forward. The PkgEntry and RegistryInstance types may have the same name, but their fields are just different enough to cause trouble.

I tested this locally on Julia 1.6 and 1.7, but I'd appreciate an extra set of eyes to make sure I didn't mess anything up.

[mattBrzezinski]

This package had its workflows disabled, things look good to me though. Not sure how to re-kick the CI to run, maybe just this comment will be enough? If not could you amend a commit message, force push back and that should start the job?

[danielmatz]

I force pushed, but now it seems to be waiting for your approval.

[giordano]

Yet another option is to use https://github.com/GunnarFarneback/RegistryInstances.jl (and this package was actually already suggested as a possible beneficiary)

[danielmatz]

I finally had some time to look at this again. I've created yet another pull request that takes the approach of using RegistryInstances. See #42. I'll close this in favor of that new PR.