[cfe] Add _BuilderRegistry

This avoid creating _AddBuilder and instead registers created builders
directing into the name space being created.

Change-Id: Ice2d98d3ab5549d955b30106597bf66512fa6849
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436220
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther