Start building the scheduler for Windows

Building derivations is a lot harder, but the downloading goals is
portable enough.

The "common channel" code is due to Volth. I wonder if there is a way we
can factor it out into separate functions / files to avoid some
within-function CPP.

Co-authored-by: volth <volth@volth.com>

src/libutil/error.hh

@@ -206,11 +206,11 @@ MakeError(SystemError, Error);
  *
  * Throw this, but prefer not to catch this, and catch `SystemError`
  * instead. This allows implementations to freely switch between this
- * and `WinError` without breaking catch blocks.
+ * and `windows::WinError` without breaking catch blocks.
  *
  * However, it is permissible to catch this and rethrow so long as
  * certain conditions are not met (e.g. to catch only if `errNo =
- * EFooBar`). In that case, try to also catch the equivalent `WinError`
+ * EFooBar`). In that case, try to also catch the equivalent `windows::WinError`
  * code.
  *
  * @todo Rename this to `PosixError` or similar. At this point Windows
@@ -248,7 +248,9 @@ public:
 };
 
 #ifdef _WIN32
-class WinError;
+namespace windows {
+    class WinError;
+}
 #endif
 
 /**
@@ -258,7 +260,7 @@ class WinError;
  */
 using NativeSysError =
 #ifdef _WIN32
-    WinError
+    windows::WinError
 #else
     SysError
 #endif