lib/scanner: Use fs.Filesystem for all operations

One more step on the path of the great refactoring. Touches rwfolder a
little bit since it uses the Lstat from fs as well, but mostly this is
just on the scanner as rwfolder is scheduled for a later refactor.

There are a couple of usages of fs.DefaultFilesystem that will in the
end become a filesystem injected from the top, but that comes later.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4070
LGTM: AudriusButkevicius, imsodin

lib/fs/filesystem.go

@@ -7,8 +7,9 @@
 package fs
 
 import (
-	"errors"
 	"io"
+	"os"
+	"path/filepath"
 	"time"
 )
 
@@ -37,6 +38,7 @@ type File interface {
 	io.WriterAt
 	io.Closer
 	Truncate(size int64) error
+	Stat() (FileInfo, error)
 }
 
 // The FileInfo interface is almost the same as os.FileInfo, but with the
@@ -57,12 +59,20 @@ type FileInfo interface {
 // FileMode is similar to os.FileMode
 type FileMode uint32
 
+// ModePerm is the equivalent of os.ModePerm
+const ModePerm = FileMode(os.ModePerm)
+
 // DefaultFilesystem is the fallback to use when nothing explicitly has
 // been passed.
-var DefaultFilesystem Filesystem = new(BasicFilesystem)
+var DefaultFilesystem Filesystem = NewBasicFilesystem()
 
 // SkipDir is used as a return value from WalkFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
 // as an error by any function.
-var errSkipDir = errors.New("skip this directory")
-var SkipDir = errSkipDir // silences the lint warning...
+var SkipDir = filepath.SkipDir
+
+// IsExist is the equivalent of os.IsExist
+var IsExist = os.IsExist
+
+// IsNotExist is the equivalent of os.IsNotExist
+var IsNotExist = os.IsNotExist