pkg/macos: clean up for Zig 0.14, consolidate C imports into one decl

Fixes #6727

The major change in this commit is to consolidate all the C imports in
a single decl in main.zig. This is required for Zig 0.14. Without it,
the problem in #6727 will happen. I was never able to minimize why this
happens in order to open a Zig bug.

Beyond this, I fixed the build.zig and build.zig.zon to work with Zig
0.14 so that we can test building `pkg/macos` in isolation. There are no
downstream impacting changes in the build.zig files.

pkg/macos/dispatch/c.zig

@@ -1,3 +1 @@
-pub const c = @cImport({
-    @cInclude("dispatch/dispatch.h");
-});
+pub const c = @import("../main.zig").c;