func (g *FileCopyGetter) GetFile(dst string, u *url.URL) error {
	underlying := &getter.FileGetter{Copy: true}
	return underlying.GetFile(dst, u)
}
The original FileGetter already knows how to do file copying so long as we set the Copy flag to true, so just
delegate to it
package cli
import (
	"fmt"
	"net/url"
	"os"
	"github.com/gruntwork-io/terragrunt/util"
	"github.com/hashicorp/go-getter"
)
const SOURCE_MANIFEST_NAME = ".terragrunt-source-manifest"
type FileCopyGetter struct {
	getter.FileGetter
}
func (g *FileCopyGetter) Get(dst string, u *url.URL) error {
	path := u.Path
	if u.RawPath != "" {
		path = u.RawPath
	}
	if fi, err := os.Stat(path); err != nil {
		return fmt.Errorf("source path error: %s", err)
	} else if !fi.IsDir() {
		return fmt.Errorf("source path must be a directory")
	}
	return util.CopyFolderContents(path, dst, SOURCE_MANIFEST_NAME)
}