Speed up SettingsBuilder (#1221)

* Speed up SettingsBuilder

It's unnecessary to build up a whole grouped dictionary only to check
if all platforms are identical and then immediately discard the
dictionary.

Instead we can check if all targets match the first platform, which
avoids creating a new dictionary but also allows bailing early as soon
as a non-matching platform is found.

Generating a large project (36MB json spec) on an M1 Max machine leads
to a ~6% total speedup: 28.48s vs 30.07s.

* Add changelog entry