Fix 'gulp css' and remove the now redundant 'gulp build --css-only'

[rsimha]

This PR does the following:

1. Makes sure that gulp css also builds CSS within extensions
2. Removes the now redundant --css-only flag to gulp build, now that there's no difference between gulp css and gulp build --css-only
3. Updates gulp dist to not need any prerequisite css build steps. You can now do gulp clean followed by gulp dist
4. Updates gulp watch to not need any prerequisite css build steps, and fixes it so it's not missing any dependencies

Fixes #9640