text: Title: Don't log warning for "Couldn't load adapter" if adapter isn't specified
  Body: Currently I get a bunch of warnings like:
  ```
  Couldn't load adapter datasetteapi = shillelagh.adapters.api.datasette:DatasetteAPI
  ```
  even though I am explicitly passing in a list of adapters and not specifying that adapter.
  
  These warnings should be printed only if the adapter is in the `adapters` list / there is no list:
  https://github.com/betodealmeida/shillelagh/blob/a427de0b2d1ac27402d70b8a2ae69468f1f3dcad/src/shillelagh/backends/apsw/db.py#L510-L511