Allow nested collections (#1400)

[distantnative]

Describe the PR

Allows collection files to be placed inside the collections folder as well as one subdirectory lower:

collections/
∟ folder/
  ∟ name.php

to be called by $kirby->collection('folder/name').

Related issues

• Fixes [Cms] Nested folders for collection() method #1400

[bastianallgeier]

We can't use GLOB_BRACE as it is not available on Windows :( I also think that I didn't implement collection loading in a good way. It shouldn't be necessary to scan the collections folder in order to get a single collection. It would be better if we refactor it first and then implement the feature.

[distantnative]

Let's rather reopen this and keep working on it ;)
I thought about only loading an individual collection on the ->get() call, but didn't want to change too much. But in that case then let's rewrite the whol load part.

[distantnative]

@bastianallgeier Ping - I added a suggestion how to refactor the class, only loading a collection file once it is requested.