Better build orchestration #483

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:
   

   ruby.wasm/lib/ruby_wasm/build/product/baseruby.rb

   Line 26 in a5d06a0

   return if Dir.exist?(install_dir)

   ruby.wasm/lib/ruby_wasm/build/product/crossruby.rb

   Line 171 in a5d06a0

   if !File.exist?("#{build_dir}/Makefile") || reconfigure

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.
   

   ruby.wasm/lib/ruby_wasm/build.rb

   Line 78 in a5d06a0

   def cache_key(digest)

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.