Emit one "nightly feature use" error per feature, not per use

If using std::sync::nonpoison::rwlock::RwLock for example without #![feature(nonpoison_rwlock)], we get one error per mention of RwLock in the project. We should emit a single error per feature, at least on a per scope basis. This will reduce verbosity significantly when a project that targets a newer MSRV where these features have been stabilized is being built on an older rustc.

Start by reproducing the example using std::sync::nonpoison::rwlock::RwLock without #![feature(nonpoison_rwlock)] and observe the repeated diagnostics. No source file or test is named in the issue, so trace the compiler path that emits nightly feature-use errors. Done means a feature produces one error per scope rather than one error per mention.