Move collections to default namespace

Currently collections are referenced in the default namespace but referencing concepts use a separate collection-namespace (<DEFAULT_NAMESPACE>/collections). This PR removes the collection-namespace and strictly uses the default namespace for collections.

## Example:

```
# a collection
@prefix : <http://try.iqvoc.net/>.
@prefix skos: <http://www.w3.org/2004/02/skos/core#>.

:indoors a skos:Collection;  # <------- collection in default namespace
         skos:prefLabel "indoors"@en;
         skos:member :audiophilia;
...
```

```
# a concept
@prefix : <http://try.iqvoc.net/>.
@prefix coll: <http://try.iqvoc.net/collections/>.
@prefix schema: <http://try.iqvoc.net/schema#>.
@prefix skos: <http://www.w3.org/2004/02/skos/core#>.

:achievement_hobbies skos:prefLabel "Achievement hobbies"@en.
:amateur_radio a skos:Concept;
               schema:memberOf coll:indoors;  # <------- Parent collection in :coll-namespace
               skos:inScheme :scheme;
               skos:prefLabel "Amateur radio"@en;
               skos:broader :achievement_hobbies.
```