Use namespaced loaders rather than method with global scope

[pyrmont]

Rouge currently uses two global methods, #load_relative and #lexer_dir, defined in lib/rouge.rb to load the various files that constitute Rouge. Rather than pollute the global namespace, it would be preferable if the loading methods were within the Rouge module.

This PR adds two top-level methods to Rouge's singleton class: #load_file and #load_lexers. It is a fork of, and builds on the work of @ashmaroli in, #1430.

[ashmaroli]

Thank you for taking on this @pyrmont
I know that it is going to be insignificant but as an FYI, it is always better (memory-allocation wise) to provide as little parameters as possible without affecting readability to File.join especially since / is the default separator on all platforms including Windows.
i.e.:
File.join(variable, 'foo', 'bar', 'lipsum') will allocate more strings than
File.join(variable, 'foo/bar/lipsum')

[pyrmont]

@ashmaroli Ta! Do you think it needs anything else?

[ashmaroli]

Technically, # @api private has no significance but to serve as a note to the adventurous..
How about moving all the load_* calls under the Rouge singleton class. That way, the load_* methods could be truly marked as private ..

module Rouge
  class << self
    def reload!
      ...
    end

    private

    def load_file(path)
      ...
    end

    load_file 'version'
  end
end

In theory, it should work seamlessly with Rouge's existing workflow. However, I've not tested it and therefore if you feel it is unnecessary or if it won't work in practice, the pull request as of this comment looks good to be merged.

[pyrmont]

@ashmaroli We can't actually make the methods private because they're called from outside Rouge (technically you could still call them using Object#send but obviously that's not something we want to be doing). While marking the methods as @api private doesn't prevent anyone else from using them, I think it's a decent compromise.

I did make one more change, though. Given that the methods are not intended to be part of a public API, I've moved them below the #reload! and #highlight methods so the intent is a little clearer structurally.

Thanks for the feedback and the initial PR!

[ashmaroli]

because they're called from outside Rouge

Yeah that's why I suggested moving the calls inside the Rouge singleton class in my comment.
Either ways, let us run with the current implementation for some time.
Thanks for handling this @pyrmont

[pyrmont]

Ah, I see what you meant. Sorry about that—my misunderstanding. Agreed to see how this goes for now :)