builder, compiler: compile and cache packages in parallel

This commit switches from the previous behavior of compiling the whole
program at once, to compiling every package in parallel and linking the
LLVM bitcode files together for further whole-program optimization.
This is a small performance win, but it has several advantages in the
future:

  - There are many more things that can be done per package in parallel,
    avoiding the bottleneck at the end of the compiler phase. This
    should speed up the compiler futher.
  - This change is a necessary step towards a non-LTO build mode for
    fast incremental builds that only rebuild the changed package, when
    compiler speed is more important than binary size.
  - This change refactors the compiler in such a way that it will be
    easier to inspect the IR for one package only. Inspecting this IR
    will be very helpful for compiler developers.

src/internal/task/task_coroutine.go

@@ -77,22 +77,9 @@ func Current() *Task
 // This is implemented inside the compiler.
 func Pause()
 
-type taskHolder interface {
-	setState(*rawState) *rawState
-	returnTo(*rawState)
-	returnCurrent()
-	setReturnPtr(unsafe.Pointer)
-	getReturnPtr() unsafe.Pointer
-}
-
-// If there are no direct references to the task methods, they will not be discovered by the compiler, and this will trigger a compiler error.
-// Instantiating this interface forces discovery of these methods.
-var _ = taskHolder((*Task)(nil))
-
 func fake() {
 	// Hack to ensure intrinsics are discovered.
 	Current()
-	go func() {}()
 	Pause()
 }