Stop registering Swift toolchains transitively

[dzbarsky]

Summary

It's bad practice for rules to autoregister toolchains, as it takes away control from end users and can cause undesired fetches. Especially when the toolchain in question triggers non-hermetic repo rule executions.

Mark //swift/toolchains:all registration in rules_swift's MODULE.bazel as a development dependency. Dependent Bazel modules no longer inherit the autoconfigured Swift toolchain registration and must explicitly register @rules_swift//swift/toolchains:all.

Update README.md and .github/generate-notes.sh so release snippets include the required explicit registration. rules_swift development continues to register //swift/toolchains:all because rules_swift is the root module during development.

[keith]

which case is this eagerly fetched now vs your override (if you need it at all) taking over?

[dzbarsky]

My primary interest is making sure the pattern fixed in #1651 isn't accidentally reintroduced. But also in Adins PR there are repeated references to needing to register toolchains in the right order so they don't override each other. I think it's simplest to just let the consumer issue the 1-line call and have full control :)