I think this makes a lot of sense (and read the proposal and was part of the pitch discussion), and this is inline with similar previous features and facilitates module-by-module migration, which is great. So +1 for this proposal, no reservations really.
One small comment (for both this proposal and future ones) - its in general great to add a use-site example of new API proposed even if easy/straightforward, it helps a lot in cognitive load navigating what it's all about and is much appreciated - just a friendly suggestion for the future. 
I do still think that the default toolchain language mode selection (when not using SwiftPM and selecting tools/language versions there) is a bit strange as it seems we are stuck at Swift 5 as the default for all eternity? But that's for another discussion - so anyone interested in that can follow up there.