This package contains elements of the runtime which are not user visible parts of the API. It is a pure source package that does no native initialization by default (unlike the runtime package which initializes the native library as part of exporting its API).
This distinction is important because some runtime libraries (like tracy will attempt to connect to a profiler on load and there are cases where we want to do things without triggering such load-time behavior).
As peers to this package, there are native _runtime_libs* packages for different variations of native libraries and tools.