Implementing an initial `Finddispatch.cmake` module to find Dispatch
from the system SDK on Apple platforms. Still should implement a basic
mechanism for finding it on Windows and Linux. Note that those platforms
will likely use the dispatch config mechanism most frequently, but this
should still work as a fallback.
This mechanism is the only one that works for Apple platforms though as
those use libdispatch from the SDK rather than building it with
corelibs-libdispatch.
When building for Linux and Windows, it is possible to build a custom
corelibs-libdispatch runtime with `dispatch_DIR` and avoid this module.
This module makes it possible to use a pre-built corelibs-libdispatch
when one is available without needing to build it. The module uses
the `Swift_SDKROOT` filepath as a hint on where to look for the runtime
libraries and the `dispatch_STATIC` boolean to specify whether it should
find the dynamic library or the static archive.
