entryPoint information in metafile

[remorses]

Close #711

Adds a entryPoint field in the metafile

{
  "outputs": {
    "react/index.js": {
      "entryPoint": "src/entry.js",
      "exports": ["default"],
      "imports": [
        ...
      ],
    
    }
}

[evanw]

Not sure we want to use this name, maybe inputEntrypoint is better?

Yeah "facade" doesn't tell me anything about what this property means. Maybe just entryPoint to mirror the entryPoints API option?

Note to self: This PR doesn't mark CSS files generated by importing CSS from a JavaScript entry point with this property. However, I think that is actually a good idea because that keeps it a 1:1 mapping. And in the future, I may need to generate more than one CSS file in this case so annotating the CSS files with this property is potentially confusing. I think I should instead add a new property to link the JavaScript entry point to the necessary CSS files instead.