build: ship flat es modules

[devversion]

Similar to angular/angular (see changelog), Angular Material should start shipping a flat ESM file.

• See https://unpkg.com/@angular/core@4.0.0-rc.1/

At first glance, it looks like this would require some project re-structuring. But there might be also alternate approaches to archive the same release structure.

@kara Just confirming again, is this something we should do on Material as well?

[devversion]

See #3751

[angular-automatic-lock-bot]

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