Webpackerのディレクトリ構成をRailsから切り離す形に変える方法 ref: http://qiita.com/y-temp4/items/917fd541676bd04b78a9

.gitignore

# See https://help.github.com/articles/ignoring-files for more about ignoring files.
#
# If you find yourself ignoring temporary files generated by your text editor
# or operating system, you probably want to add a global ignore instead:
#   git config --global core.excludesfile '~/.gitignore_global'

# Ignore bundler config.
/.bundle

# Ignore all logfiles and tempfiles.
/log/*
/tmp/*
!/log/.keep
!/tmp/.keep

/node_modules
/yarn-error.log

.byebug_history
+/public/entry
-/public/packs
/node_modules

vendor/bundle

application.js

/* eslint no-console:0 */
// This file is automatically compiled by Webpack, along with any other files
// present in this directory. You're encouraged to place your actual application logic in
// a relevant structure within app/javascript and only use these pack files to reference
// that code so it'll be compiled.
//
// To reference this file, add <%= javascript_pack_tag 'application' %> to the appropriate
// layout file, like app/views/layouts/application.html.erb
import 'stylesheets/application'
import 'javascripts/hello_react.jsx'

console.log('Hello World from Webpacker')

application.scss

body {
  background: red;
}

file0.txt

frontend
├── entry
├── javascripts
└── stylesheets

file8.erb

<%= javascript_pack_tag 'application' %>
<%= stylesheet_pack_tag 'application' %>

manifest.js

{
  "application.css": "http://localhost:8080/entry/application.css",
  "application.css.map": "http://localhost:8080/entry/application.css.map",
  "application.js": "http://localhost:8080/entry/application.js",
  "application.js.map": "http://localhost:8080/entry/application.js.map"
}

paths.yml

default: &default
  config: config/webpack
- entry: packs
+ entry: entry
  output: public
  manifest: manifest.json
  node_modules: node_modules
- source: app/javascript
+ source: frontend
  extensions:
    - .coffee
    - .js
    - .jsx
    - .ts
    - .vue
    - .sass
    - .scss
    - .css
    - .png
    - .svg
    - .gif
    - .jpeg
    - .jpg

development:
  <<: *default

test:
  <<: *default
  manifest: manifest-test.json

production:
  <<: *default

shared.js

  resolve: {
    extensions: paths.extensions,
    modules: [
      resolve(paths.source),
      resolve(paths.node_modules)
    ]
  },