Remove bufferline folder

[litoj]

I think it is unnecessary to have a copy of an entire plugin just beacause of a rename. In case you really want to avoid forcing users to use the actual plugin name you could use a loop for all bufferline.xxx and put a hook in package.preload['bufferline.xxx'] that would return the corresponding barbar.xxx module.

[Iron-E]

I'm not immediately familiar with that solution, though I can certainly look into it (since it sounds much more clean than what we're doing).

A PR would be welcome if you are open to it and get around to it faster!

[litoj]

I opened up a PR that keeps it working exactly the way it was now.

[romgrk]

I've added this point to the 2.0 issue, at some point we should have users migrate to require'barbar'.

[Iron-E]

Will close this as it's being tracked in #394.