Divert working files to a different directory (than the build one)

[izaera]

Bundler 3 is generating files in three directories:

1. build/output: real output
2. build/generated: generated JS files and webpack configuration
3. build/webpack: output from webpack

In order to maintain compatibility with bundler 2 we may want to map output as:

1. build
2. work/generated
3. work/webpack

So we'll provide a work-dir config option to do this.

[wincent]

For the lay person (ie. me), can you clarify what the difference between "generated" and "real output" is?

[izaera]

Generated are JS files that bundler 3 created to feed them to webpack and make everything glue together. Right now they are:

1. webpack configuration (it is created in memory and passed to webpack, but the bundlers writes it for diagnostic purposes only)
2. export entry point JS files: one file per entry point where we simply do a require to bridge AMD and webpack's bundle.

An example of 2 would be:

__MODULE__.exports = require('recharts');

This "magic" code will be processed by webpack and the require will end up transformed to a webpack_require that resolves to a module defined inside the webpack bundles.

The MODULE on the other hand, will be bound by the bundler (when AMD-wrapping the webpack bundles) in the final output so that the return from webpack_require is exported through our AMD loader.

It' the same scheme we use when adapting native framework apps.