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.go

@@ -12,8 +12,6 @@ import (
 	"encoding/binary"
 	"fmt"
 	"strconv"
-
-	"golang.org/x/sys/windows"
 )
 
 // Variant specifies which GUID variant (or "type") of the GUID. It determines
@@ -39,13 +37,6 @@ type Version uint8
 var _ = (encoding.TextMarshaler)(GUID{})
 var _ = (encoding.TextUnmarshaler)(&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 so that stringification and
-// marshaling can be supported. The representation matches that used by native
-// Windows code.
-type GUID windows.GUID
-
 // NewV4 returns a new version 4 (pseudorandom) GUID, as defined by RFC 4122.
 func NewV4() (GUID, error) {
 	var b [16]byte

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
+}

pkg/guid/guid_windows.go

@@ -0,0 +1,10 @@
+package guid
+
+import "golang.org/x/sys/windows"
+
+// 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 so that stringification and
+// marshaling can be supported. The representation matches that used by native
+// Windows code.
+type GUID windows.GUID