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/lstat_test.go

@@ -0,0 +1,25 @@
+package system
+
+import (
+	"testing"
+)
+
+func TestLstat(t *testing.T) {
+	file, invalid, _ := prepareFiles(t)
+
+	statFile, err := Lstat(file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if statFile == nil {
+		t.Fatal("returned empty stat for existing file")
+	}
+
+	statInvalid, err := Lstat(invalid)
+	if err == nil {
+		t.Fatal("did not return error for non-existing file")
+	}
+	if statInvalid != nil {
+		t.Fatal("returned non-nil stat for non-existing file")
+	}
+}