Allow guid package to be used on non-Windows GOOS targets

`golang.org/x/sys/windows.GUID` is currently only available to builds
targeting `GOOS=windows` (see golang/go#36485). This change makes it
so `windows` builds continue to use `golang.org/x/sys/windows.GUID`,
while non-`windows` builds get a different structure that has all
the same fields and methods.

pkg/guid/guid_nonwindows.go

@@ -0,0 +1,15 @@
+// +build !windows
+
+package guid
+
+// GUID represents a GUID/UUID. It has the same structure as
+// golang.org/x/sys/windows.GUID so that it can be used with functions expecting
+// that type. It is defined as its own type as that is only available to builds
+// targeted at `windows`. The representation matches that used by native Windows
+// code.
+type GUID struct {
+	Data1 uint32
+	Data2 uint16
+	Data3 uint16
+	Data4 [8]byte
+}