refactor(@angular-devkit/core): don't use concrete type as interface

[jkrems]

Set isn't a stable interface, it's a concrete type that changes over time (new spec versions may add methods, older ones may lack methods). There shouldn't be a huge downside to just declaring this class as "its own thing".

Follow-up to #30208 which breaks on google3 import because the Set types are subtly different because of different TS configs.

[jkrems]

The changes were merged into the following branches: main, 20.0.x

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}