Refactor pkg/archive with a platform-independent stat struct

pkg/archive contains code both invoked from cli (cross platform) and
daemon (linux only) and Unix-specific dependencies break compilation on
Windows. We extracted those stat-related funcs into platform specific
implementations at pkg/system and added unit tests.

Signed-off-by: Ahmet Alp Balkan <ahmetb@microsoft.com>

system/stat_windows.go

@@ -0,0 +1,12 @@
+// +build windows
+
+package system
+
+import (
+	"errors"
+	"syscall"
+)
+
+func fromStatT(s *syscall.Win32FileAttributeData) (*Stat, error) {
+	return nil, errors.New("fromStatT should not be called on windows path")
+}