Use RegistryInstances

[mortenpi]

I'm not actually that knowledgeable about RegistryInstances, but I think (hope) that it would allow us to get rid of all the custom registry parsing we do here:

DocumentationGenerator.jl/src/utils/pkgtools.jl

Lines 312 to 354 in e553973

	function installable_on_version(reg::UnifiedRegistryInfo, version)
	allpkgs = Dict()
	pkgpaths = map(x -> x.metadir, collect(values(reg.info)))
	filter!(isdir, pkgpaths)
	if isempty(pkgpaths)
	@error "No package folders found inside the registry"
	return allpkgs
	end
	for pkg in pkgpaths
	"Compat.toml" in readdir(pkg) || continue
	pkgtoml = Pkg.TOML.parsefile(joinpath(pkg, "Package.toml"))
	versions = Pkg.TOML.parsefile(joinpath(pkg, "Versions.toml"))
	compat = Pkg.TOML.parsefile(joinpath(pkg, "Compat.toml"))
	compatible_versions = VersionNumber[]
	for pkgver in keys(compat)
	try
	if haskey(compat[pkgver], "julia")
	if any(in.(version, Pkg.Types.VersionRange.(compat[pkgver]["julia"])))
	append!(compatible_versions, [
	VersionNumber(v) for v in keys(versions) if
	VersionNumber(v) in Pkg.Types.VersionRange(pkgver)
	])
	end
	else
	append!(compatible_versions, [
	VersionNumber(v) for v in keys(versions) if
	VersionNumber(v) in Pkg.Types.VersionRange(pkgver)
	])
	end
	catch err
	@error err
	end
	end
	allpkgs[pkgtoml["uuid"]] = (
	name = pkgtoml["name"],
	url = pkgtoml["repo"],
	uuid = pkgtoml["uuid"],
	path = pkg,
	versions = compatible_versions,
	)
	end
	allpkgs
	end

And also it would hopefully support the zipped up registry format out of the box as well.