Simplify build and js serving file structures

[kpdecker]

A number of different changes here that are unfortunately tied.

1. Move all client source into .next/bundles dir
2. Simplify serving logic where possible to map directly to these paths
3. Define buildId at the start of the build and tag dynamic chunk loading with this.
4. Avoid creating duplicate chunks with the same content when loaded via different explicit require paths.

These changes also make it much easier for CDN users to simply copy the relevant content as static files rather than having to do manual transforms on the files.

Potentially breaking changes:

1. Requests for page javascript are now *.js. / is special cased to be index.js.
2. Composite source files are not filtered based on dev or not
3. _error/index.js -> _error.js

[arunoda]

Thanks for this.
I'd like to see some of these changes, but I won't accept any directory structure changes. (At this stage, may be in the future)

But I'd love to see webpack changes right now. But send them as different PRs.

[kpdecker]

@arunoda can you be explicit on which items are the webpack changes so I can properly cut scope?

[kpdecker]

Also, what are the concerns on the directory structure? Regarding stage, are you referencing for a 3.x vs. 4.x?