refactor fields into a package

orm/__init__.py

@@ -6,13 +6,13 @@ from orm.fields import (
     DateTime,
     Decimal,
     Float,
-    ForeignKey,
     Integer,
     JSON,
     String,
     Text,
     Time,
 )
+from orm.fields.foreign_key import ForeignKey
 from orm.models import Model
 
 __version__ = "0.0.1"
@@ -28,7 +28,6 @@ __all__ = [
     "Date",
     "Decimal",
     "Float",
-    "ForeignKey",
     "Model",
     "ModelDefinitionError",
     "ModelNotSet",