Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/driveletter/driveletter.go

@@ -0,0 +1,14 @@
+// Package driveletter returns whether a name is a valid drive letter
+
+// +build !windows
+
+package driveletter
+
+// IsDriveLetter returns a bool indicating whether name is a valid
+// Windows drive letter
+//
+// On non windows platforms we don't have drive letters so we always
+// return false
+func IsDriveLetter(name string) bool {
+	return false
+}