Include all available platforms for a gem in the gemset

gemset attrs stay the same, with the addition of a nativeSources set:

```nix
{
  libv8-node = {
    groups = ["default"];
    nativeSources = [{
      platform = "arm64-darwin-20";
      remotes = ["https://rubygems.org"];
      sha256 = "04qnpw0bm8j82km2whx5zhw59lm8isd3i540g1xfyc0mqw0vynhp";
      type = "gem";
    } {
      platform = "x86_64-darwin-20";
      remotes = ["https://rubygems.org"];
      sha256 = "0r65dwxjrk6s9q4lf1jf7kg344qg3fn8d9r6nad91swmabpzd2bg";
      type = "gem";
    } {
      platform = "x86_64-linux";
      remotes = ["https://rubygems.org"];
      sha256 = "04avzccvrjk4nbi7926cvbdkpkgc6a2m0gyz1s1k4x7q06lkdnnk";
      type = "gem";
    }];
    platforms = [];
    source = {
      platform = "ruby";
      remotes = ["https://rubygems.org"];
      sha256 = "0k2cqxnbm7lm284fa65bf4b18w7k6977mh6anyyai34r43g3vm34";
      type = "gem";
    };
    version = "15.14.0.0";
  };
}
}
```

Author: jdelStrother

lib/bundix.rb

@@ -40,15 +40,27 @@ def convert
     lock = parse_lockfile
     dep_cache = build_depcache(lock)
 
+    gems = Hash.new { |h, k| h[k] = [] }
+
     # reverse so git comes last
-    lock.specs.reverse_each.with_object({}) do |spec, gems|
+    lock.specs.reverse_each do |spec|
       gem = find_cached_spec(spec, cache) || convert_spec(spec, cache, dep_cache)
-      gems.merge!(gem)
-
+      gem = gem.values.first # `gem` is a hash with only one value
       if spec.dependencies.any?
-        gems[spec.name]['dependencies'] = spec.dependencies.map(&:name) - ['bundler']
+        gem['dependencies'] = spec.dependencies.map(&:name) - ['bundler']
       end
+      gems[spec.name] << JSON.load(JSON.dump(gem)) # awful hack to stringify keys
     end
+
+    gems.map do |name, variants|
+      # use the plain old ruby gem as the primary source, add other platforms as 'nativeSources'
+      # If for some reason there's no plain ruby gem, just use the first available platform as the primary source
+      primary, *others = variants.sort_by do |v|
+        platform = v["source"]["platform"] || "ruby"
+        [platform == "ruby" ? 0 : 1, platform]
+      end
+      [name, primary.merge("nativeSources" => others.map { |o| o["source"] }.uniq)]
+    end.to_h
   end
 
   def groups(spec, dep_cache)
@@ -100,6 +112,10 @@ def convert_spec(spec, cache, dep_cache)
   end
 
   def find_cached_spec(spec, cache)
+    if spec.platform != "ruby"
+      puts "TODO: figure out a sane way of dealing with nativeSources from the cache"
+      return nil
+    end
     name, cached = cache.find{|k, v|
       next unless k == spec.name
       next unless cached_source = v['source']

lib/bundix/source.rb

@@ -101,7 +101,7 @@ def fetch_local_hash(spec)
         if has_platform
           # Find first gem that matches the platform
           platform = File.basename(path, '.gem')[(name_version.size + 1)..-1]
-          next unless Gem::Platform.match(platform)
+          next unless spec.platform =~ platform
         end
 
         hash = nix_prefetch_url(path)[SHA256_32]
@@ -126,7 +126,7 @@ def fetch_remote_hash(spec, remote)
         # Fetch remote spec to determine the exact platform
         # Note that we can't simply use the local platform; the platform of the gem might differ.
         # e.g. universal-darwin-14 covers x86_64-darwin-14
-        spec = spec_for_dependency(remote, spec.name, spec.version)
+        spec = spec_for_dependency(remote, spec)
         return unless spec
       end
 
@@ -141,9 +141,15 @@ def fetch_remote_hash(spec, remote)
       nil
     end
 
-    def spec_for_dependency(remote, name, version)
+    def spec_for_dependency(remote, dependency)
       sources = Gem::SourceList.from([remote])
-      Gem::SpecFetcher.new(sources).spec_for_dependency(Gem::Dependency.new(name, version)).first&.first&.first
+      specs, _errors = Gem::SpecFetcher.new(sources).spec_for_dependency(Gem::Dependency.new(dependency.name, dependency.version), false)
+      specs.each do |spec, source|
+        return spec if dependency.platform == spec.platform
+      end
+      # TODO: When might this happen?
+      puts 'oh, fallback ' + dependency.platform.to_s
+      specs.first.first
     end
   end
 
@@ -191,6 +197,7 @@ def convert_rubygems
           type: 'gem',
           remotes: (remote ? [remote] : remotes),
           sha256: hash,
+          platform: platform || "ruby",
         },
       }
     end