Workers -> WorkersES6, Workers/Build -> Workers

[mramato]

Move Worker code into WorkersES6 and build Workers into the Workers directory. This is so we (and users) don't have to do string manipulation to rename Workers/Build -> Workers when building ES6 based apps that rely on source modules.

[cesium-concierge]

Thanks for the pull request @mramato!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

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