Check for collections_dir

[DirtyF]

related to jekyll/jekyll#6685

Given `collections_dir` is defined in config
Then `jekyll post` should create new post in `collection_dir/_posts`
And `jekyll draft` should create new draft in `collections_dir/_drafts`
And `jekyll publish` should move draft in `collection_dir/_posts`
And `jekyll unpublish` should move post in `collections_dir/_drafts`

[johnlane]

I am not sure if this is related, but...

When _drafts and _posts are under the collection directory (instead of the root) and a draft is published, it is moved from collection_dir/_drafts/foo.md to _posts/yyyy-mm-dd-foo.md.

It should move to collection_dir/_posts/yyyy-mm-dd-foo.md.

(I'm using 3.7.2)