-enable-experimental-private-intransitive-dependencies -> -enable-direct-intramodule-dependencies
-disable-experimental-private-intransitive-dependencies -> -disable-direct-intramodule-dependencies
While we're here, rename DependencyCollector::Mode's constants and clean
up the documentation.
The old tests were just running the type checker because we used to only emit reference dependencies after Sema. Now that we emit them after the pipeline has run, let's upgrade these tests to capture these new references.
Add a mode bit to the dependency collector that respects the frontend flag in the previous commit.
Notably, we now write over the dependency files at the end of the compiler pipeline when this flag is on so that dependency from SILGen and IRGen are properly written to disk.
