🏗♻️ Refactor and speed up unminified builds

[rsimha]

This PR does the following:

• Cleans up the code used to compile unminified JS
• Simplifies the handling of --watch (Future cleanup: Simplify the two-step build added in No longer recursively install watchers on extensions. #47)
• Fixes build timing by starting measurement when bundling actually begins (Add step timing for gulp build commands #10290 (review))
• Stops using lazypipe (added in add lazypipes to consolidate some repetition on stream chaining #702, but slows down the build now that we have ~150 targets)
• Stops modifying _init_tests.js during watch to trigger tests (added in Re-bundle karma test files when non-test JS files change. #1667, no longer necessary)
• Speeds up unminified builds by ~15%

Running time for gulp build on Travis: 5m 35s → 4m 42s

Follow up to #47, #702, #1667, #10290, and #22254