Writing a new jekyll command without an external gem

I’d like to write a new jekyll command which will have some custom workflow around managing documents within a jekyll site). It looks straightforward except it doesn’t appear to work if the command code

class ProposalCommands < Jekyll::Command
  class << self
    def init_with_program(prog)

is placed in the _plugins folder. It appears that I need to create a gem, deploy it and then add it to my Gemfile. Since this code is very specific to this one project that seems a little overkill.
Is there a way to develop code within the main project before deploying gems to a repository?