Add Finddispatch module

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.

Author: etcwilde

Runtimes/Core/cmake/modules/Finddispatch.cmake

@@ -0,0 +1,67 @@
+#[=======================================================================[.rst:
+Finddispatch
+------------
+
+Find libdispatch on macOS and Linux while deferring to dispatchConcig.cmake when
+available or requested.
+
+Components
+^^^^^^^^^^
+
+This module supports the optional component `Swift`, for use with the
+`COMPONENTS` argument of the :command:`find_package` command. This component
+defines the associated ``swiftDispatch`` IMPORT target.
+
+Imported Targets
+^^^^^^^^^^^^^^^^
+
+The following :prop_tgt:`IMPORTED` TARGETS may be defined:
+
+ ``dispatch``
+
+ ``swiftDispatch``
+
+Result Variables
+^^^^^^^^^^^^^^^^
+
+The module may set the following variables if `dispatch_DIR` is not set.
+
+ ``dispatch_FOUND``
+   true if dispatch headers and libraries were found
+
+ ``dispatch_INCLUDE_DIR``
+   the directory containing the libdispatch headers
+
+ ``dispatch_LIBRARIES``
+   the libraries to be linked
+
+
+#]=======================================================================]
+
+# If the dispatch_DIR is specified, look there instead. The cmake-generated
+# config file is more accurate, but requires that the SDK has one available.
+if(dispatch_DIR)
+  if(dispatch_FIND_REQUIRED)
+    list(APPEND args REQUIRED)
+  endif()
+  if(dispatch_FIND_QUIETLY)
+    list(APPEND args QUIET)
+  endif()
+  find_package(dispatch NO_MODULE ${args})
+  return()
+endif()
+
+if(APPLE)
+  find_path(dispatch_INCLUDE_DIR "dispatch/dispatch.h")
+  find_library(dispatch_LIBRARY NAMES "libdispatch.tbd"
+    PATH "usr/lib/system"
+    PATH_SUFFIXES system)
+
+  add_library(dispatch SHARED IMPORTED GLOBAL)
+  set_target_properties(dispatch PROPERTIES
+    IMPORTED_IMPLIB "${dispatch_LIBRARY}"
+    INTERFACE_INCLUDE_DIRECTORIES "${dispatch_INCLUDE_DIR}")
+else()
+  # TODO: Implement this for Linux and Windows
+message(FATAL_ERROR "Not implemented for platform")
+endif()