Use classmap autoloading in composer

When installing via composer, using "files" causes the entire file
to be loaded when the autoloader is initialized. Using classmap
actually uses the autoloader as a real autoloader, the files are
only loaded when the class is invoked.

See https://getcomposer.org/doc/04-schema.md#files and
https://getcomposer.org/doc/04-schema.md#classmap for more details.