fs: allow Metadata calls to be called with Directory or Object

This involved adding the Fs() method to DirEntry as it is needed in
the metadata mapper.

Unspecialised fs.Dir objects will return a new fs.Unknown from their
Fs() methods as they are not specific to any given Fs.

fs/types.go

@@ -102,9 +102,6 @@ type Object interface {
 type ObjectInfo interface {
 	DirEntry
 
-	// Fs returns read only access to the Fs that this object is part of
-	Fs() Info
-
 	// Hash returns the selected checksum of the file
 	// If no checksum is available it returns ""
 	Hash(ctx context.Context, ty hash.Type) (string, error)
@@ -117,6 +114,9 @@ type ObjectInfo interface {
 // a Dir or Object.  These are returned from directory listings - type
 // assert them into the correct type.
 type DirEntry interface {
+	// Fs returns read only access to the Fs that this object is part of
+	Fs() Info
+
 	// String returns a description of the Object
 	String() string
 
@@ -332,3 +332,28 @@ type WriterAtCloser interface {
 	io.WriterAt
 	io.Closer
 }
+
+type unknownFs struct{}
+
+// Name of the remote (as passed into NewFs)
+func (unknownFs) Name() string { return "unknown" }
+
+// Root of the remote (as passed into NewFs)
+func (unknownFs) Root() string { return "" }
+
+// String returns a description of the FS
+func (unknownFs) String() string { return "unknown" }
+
+// Precision of the ModTimes in this Fs
+func (unknownFs) Precision() time.Duration { return ModTimeNotSupported }
+
+// Returns the supported hash types of the filesystem
+func (unknownFs) Hashes() hash.Set { return hash.Set(hash.None) }
+
+// Features returns the optional features of this Fs
+func (unknownFs) Features() *Features { return &Features{} }
+
+// Unknown holds an Info for an unknown Fs
+//
+// This is used when we need an Fs but don't have one.
+var Unknown Info = unknownFs{}