Better build orchestration

The underlying build system of rbwasm build is quite messy.

1. Each build step has to have its ad-hoc up-to-date check like followings:
   https://github.com/ruby/ruby.wasm/blob/a5d06a01873130fda0f07f4401a3b018c6f1d753/lib/ruby_wasm/build/product/baseruby.rb#L26 https://github.com/ruby/ruby.wasm/blob/a5d06a01873130fda0f07f4401a3b018c6f1d753/lib/ruby_wasm/build/product/crossruby.rb#L171
2. Uses of build product and cache key computation are placed far, so we often forget to update the cache key and get a wrong cache restored.
   https://github.com/ruby/ruby.wasm/blob/a5d06a01873130fda0f07f4401a3b018c6f1d753/lib/ruby_wasm/build.rb#L78
3. The current cache mechanism cannot restore two or more artifacts produced within a single rake task job. This usually happens for PIC & non-PIC builds invoked by rake npm:ruby-head-wasm-wasi:build
4. Our cache mechanism does not allow partial cache-hit, so even if we change only crossruby source revision, we have to rebuild openssl from scratch.
5. Our cache mechanism is not sensitive to recipe changes. For instance, even if we change CFLAGS value, crossruby product will not be rebuilt.

まず lib/ruby_wasm/build.rb と、lib/ruby_wasm/build/product/baseruby.rb および crossruby.rb にあるプロダクト実装を読みます。エントリーポイント rake npm:ruby-head-wasm-wasi:build と、その PIC および非 PIC アーティファクトを追跡します。ビルドキャッシュが複数のアーティファクトと部分的なヒットを処理し、ソースリビジョンやレシピの変更があっても誤った結果を復元しないことが完了条件です。