feat: refactor services folder/module structure

Refactor services folder/module structure.

**Motivation**

While working on our services I've repeatedly encountered circular imports and a general lack of clarity regarding where to put things. The structure introduced goes a long way towards resolving those issues, setting us up for a clean structure going forward.

**Services**

Services are now in their own folder with a few files:

- `services/{service_name}/__init__.py`: init as needed, mostly empty now
- `services/{service_name}/{service_name}_base.py`: the base class for the service
- `services/{service_name}/{service_name}_{impl_type}.py`: the default concrete implementation of the service - typically one of `sqlite`, `default`, or `memory`
- `services/{service_name}/{service_name}_common.py`: any common items - models, exceptions, utilities, etc

Though it's a bit verbose to have the service name both as the folder name and the prefix for files, I found it is _extremely_ confusing to have all of the base classes just be named `base.py`. So, at the cost of some verbosity when importing things, I've included the service name in the filename.

There are some minor logic changes. For example, in `InvocationProcessor`, instead of assigning the model manager service to a variable to be used later in the file, the service is used directly via the `Invoker`.

**Shared**

Things that are used across disparate services are in `services/shared/`:

- `default_graphs.py`: previously in `services/`
- `graphs.py`: previously in `services/`
- `paginatation`: generic pagination models used in a few services
- `sqlite`: the `SqliteDatabase` class, other sqlite-specific things

invokeai/app/util/metadata.py

@@ -3,7 +3,7 @@ from typing import Optional
 
 from pydantic import ValidationError
 
-from invokeai.app.services.graph import Edge
+from invokeai.app.services.shared.graph import Edge
 
 
 def get_metadata_graph_from_raw_session(session_raw: str) -> Optional[dict]:

invokeai/app/util/step_callback.py

@@ -1,8 +1,7 @@
 import torch
 from PIL import Image
 
-from invokeai.app.models.exceptions import CanceledException
-from invokeai.app.models.image import ProgressImage
+from invokeai.app.services.invocation_processor.invocation_processor_common import CanceledException, ProgressImage
 
 from ...backend.model_management.models import BaseModelType
 from ...backend.stable_diffusion import PipelineIntermediateState