Support Package Servers for offline usage

[hogandp]

Add support for resolving dependent registries from a package server vs a git clone, this would enable this package to be used in offline environments. Believe this would involve a single function to map the registries hosted in a package server by their "repo" field to a temp directory where they are extracted. I believe the register function would end up looking something like this:

    # If the package server is set, try to resolve the registries from there first
    pkg_server_registries = get_package_server_registries()

    registry_deps_paths = map(registry_deps) do registry
        if registry in keys(pkg_server_registries)
            pkg_server_registries[registry]
        else
            LibGit2.path(get_registry(GitTools.normalize_url(registry); gitconfig=gitconfig, force_reset=force_reset, cache=cache))
        end
    end