🏗♻️ Refactor / clean up the watch logic used for extensions

[rsimha]

This PR does the following:

• Moves buildExperiments() from build-system/tasks/helpers.js to build-system/tasks/dist.js (should've been done in 🏗♻️ Split gulpfile.js into separate files #22109)
• Removes unused watch logic from buildExperiments and cleans up unused gulp task (copy-pasted in Experiments.html page for our CDN #923)
• Removes unused watch logic from buildLoginDoneVersion and cleans up unused gulp task (copy-pasted in Login dialog done HTML page #1330)
• Simplifies the watch logic for extensions by using a separate watcher to rebuild an extension when any file in its directory is modified (see No longer recursively install watchers on extensions. #47, Consolidate gulp watch and gulp build #12900, Add missing option to buildExtensions() in gulp watch / gulp build #12911, and 🏗♻️ Refactor and speed up unminified builds #22635)
• Removes the obsolete bundleOnlyIfListedInFiles option used to selectively build extensions (added in RFC: Add --files to allow running specific test files. #3091) now that we have --extensions and --extensions_from (added in Add --extensions flag to only build listed extensions #13203, Support --extensions=minimal_set & bug fix #13249, and 🏗🚀Allow building extensions from an example document #17129)

Follow up to #47, #923, #1330, #3091, #12900, #12911, #13203, #13249, #17129, #22109, and #22635

[rsimha]

Tested as follows:

• No errors while running gulp build
• No errors while running gulp dist
  • experiments.js and amp-login-done-0.1.js are built as expected
• No errors while running gulp
  • Modifying a non-extension .js file rebuilds the .js file exactly once
  • Modifying a .js file in an extension directory rebuilds the entire extension exactly once
  • Modifying a non-extension .css file copies all CSS to build/ and rebuilds amp.js exactly once
  • Modifying a .css file in an extension directory rebuilds the entire extension exactly once
• No errors while running gulp --extensions amp-foo,amp-bar
  • Modifying .js and .css files in amp-foo or amp-bar results in the rebuilds described above
• Runtime and extensions served correctly by gulp serve [--compiled]

[rsimha]

Rebased on master, tested locally, Travis is green. Merging.