build: Rename build_system -> build

Author: kateinoigakukun

Rakefile

@@ -182,7 +182,7 @@ namespace :build do
 
   desc "Download prebuilt Ruby"
   task :download_prebuilt, :tag do |t, args|
-    require "ruby_wasm/build_system/downloader"
+    require "ruby_wasm/build/downloader"
 
     release =
       if args[:tag]

lib/ruby_wasm/build.rb

@@ -0,0 +1,6 @@
+require_relative "build/build_params"
+require_relative "build/product"
+require_relative "build/toolchain"
+
+module RubyWasm
+end