fs: warn the user when using a remote name without a colon

A very common mistake for new users of rclone is to use a remote name
without a colon. This can be on the command line or in the config when
setting up a crypt backend.

This change checks to see if the user uses a path which matches a
remote name and gives an NOTICE like this if they do

    NOTICE: "remote" refers to a local folder, use "remote:" to refer to your remote or "./remote" to hide this warning

See: https://forum.rclone.org/t/sync-to-onedrive-personal-lands-file-in-localfilesystem-but-not-in-onedrive/32956

fs/config.go

@@ -29,6 +29,12 @@ var (
 		return errors.New("no config file set handler")
 	}
 
+	// Check if the config file has the named section
+	//
+	// This is a function pointer to decouple the config
+	// implementation from the fs
+	ConfigFileHasSection = func(section string) bool { return false }
+
 	// CountError counts an error.  If any errors have been
 	// counted then rclone will exit with a non zero error code.
 	//

fs/config/config.go

@@ -117,6 +117,9 @@ func init() {
 	// Set the function pointers up in fs
 	fs.ConfigFileGet = FileGetFlag
 	fs.ConfigFileSet = SetValueAndSave
+	fs.ConfigFileHasSection = func(section string) bool {
+		return LoadedData().HasSection(section)
+	}
 	configPath = makeConfigPath()
 	cacheDir = makeCacheDir() // Has fallback to tempDir, so set that first
 	data = newDefaultStorage()

fs/newfs.go

@@ -26,6 +26,9 @@ import (
 // up with drive letters.
 func NewFs(ctx context.Context, path string) (Fs, error) {
 	Debugf(nil, "Creating backend with remote %q", path)
+	if ConfigFileHasSection(path) {
+		Logf(nil, "%q refers to a local folder, use %q to refer to your remote or %q to hide this warning", path, path+":", "./"+path)
+	}
 	fsInfo, configName, fsPath, config, err := ConfigFs(path)
 	if err != nil {
 		return nil, err