runtime: move internals to `internal/runtime`

[prattmic]

Right now package runtime appears to serve two purposes:

• It contains most of the implementation of the Go runtime.
• And, it exposes the world-public API for various runtime-related functionality (runtime.GOMAXPROCS, etc).

At first glance, this seems perfectly normal and reasonable, however there is actually a third purpose:

• Expose std-internal APIs that should not be world-public. e.g., runtime.semacquire for sync.Mutex or runtime.netpollopen for os and net.

These APIs should not be public to the whole world as they are implementation details that may change. However, with the current layout there is no good way to expose these internal APIs, so we must use //go:linkname between packages, and in some cases copy entire type definitions between packages (#64549).

If we move most of the runtime implementation to internal/runtime, then it could expose a richer API surface useful to other packages in std, but nothing outside of std would be able to access them due to the rules of internal. Package runtime would remain to provide the same API, but it would be much smaller and simply import internal/runtime for most functionality.

In moving, there are also opportunities for cleanup along the lines of #51087. e.g., perhaps the timer implementation is in internal/runtime/timer, etc. Though this move isn't really a prerequisite for that; we could do that in runtime/internal/timer as well.

This change would technically be solely an implementation detail, though in practice there are likely symbols that folks depend on anyway, either via linkname (https://go.dev/cl/548235) or implicitly by looking for in profiles, stack traces, etc.

cc @golang/runtime

[mauri870]

I really like the idea of having a internal/runtime package. The runtime code always felt too "convoluted" for me. It grew a lot with time to the point it is now a sum of numerous assembly files, internal code for the GC and auxiliary runtime stuff, race, asan, msan code, and buried in the middle of all this the public runtime apis. I believe having an internal package dedicated to these bits would hopefully clean runtime enough that it will be easier to navigate and more inlined with the other standard library packages.

[gopherbot]

Change https://go.dev/cl/559076 mentions this issue: runtime,time: clean duplicate definitions time.runtimeTimer and runtime.timer

[gopherbot]

Change https://go.dev/cl/559675 mentions this issue: runtime,reflect: clean duplicate definitions runtimeSelect

[gopherbot]

Change https://go.dev/cl/559715 mentions this issue: runtime: migrate internal/syscall to internal/runtime

[gopherbot]

Change https://go.dev/cl/560155 mentions this issue: runtime: migrate internal/atomic to internal/runtime

[gopherbot]

Change https://go.dev/cl/560136 mentions this issue: syscall: call internal/runtime/syscall.Syscall6 in RawSyscall6