`ExecutorHooks.h` is now nothing to do with hooks, so rename it. Also
there are some additional functions it should declare, and a couple of
places where we've slightly messed up the boundary, for instance
`swift_task_asyncMainDrainQueue` was defined in `Task.cpp` rather than
in the executor implementations, which is wrong, so fix that too.
`CooperativeGlobalExecutor.cpp` now builds against the interface from
`ExecutorImpl.h`, rather than including the all the concurrency headers.
rdar://135380149
C++ executor implementations were `#include`ed into `GlobalExecutor.cpp`,
which makes it difficult to replace the global executor when using the
Embedded Concurrency library. Refactor things so that they build into
separate objects, which means replacing them is just a matter of writing
the relevant functions yourself.
rdar://135380149
