route partial filenames should be hashed based on file contents

[thescientist13]

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

Currently route partials for the mpa mode just get named based on their route, which as a name won't change from build to build.

const outputPath = `${outputDir}/_routes${url}`

public/
  _routes/
    about/index.html
    index.html

But, the content does and so to avoid any issues with caching, and just like with CSS / JS, we should hash these filenames based on content to be self cache busting by default.

Details

The main obstacle here is that all the content needs to be known up front because the generated routes have to include entries for all pages. Since currently this happens page by page in serialize.js, there is no guarantee of order, it's all happening in parallel.

Saved a WIP branch here.