Tidy up structure of briefcase

I had a spare fifteen minutes and decided that I should tidy up my
monorepo. The work of tidying up is not finished; this is a small step in the
right direction.

TL;DR
- Created a tools directory
- Created a scratch directory (see README.md for more information)
- Added README.md to third_party
- Renamed delete_dotfile_symlinks -> symlinkManager
- Packaged symlinkManager as an executable symlink-mgr using buildGo

gopkgs/utils/utils.go

@@ -21,6 +21,11 @@ func HomeDir() string {
 	return user.HomeDir
 }
 
+// Returns true if `info` is a symlink.
+func IsSymlink(info os.FileMode) bool {
+	return info&os.ModeSymlink != 0
+}
+
 // Return true if `path` exists and false otherwise.
 func FileExists(path string) bool {
 	if _, err := os.Stat(path); os.IsNotExist(err) {