first phase of source tree restructure

This is the first phase of a big shifting of files and directories
in the source tree.

You will need to run `pip install -e .` before the code will work again!

Here's what's in the current commit:

1) Remove a lot of dead code that dealt with checkpoint and safetensor loading.
2) Entire ckpt_generator hierarchy is now gone!
3) ldm.invoke.generator.*   => invokeai.generator.*
4) ldm.model.*              => invokeai.model.*
5) ldm.invoke.model_manager => invokeai.model.model_manager

6) In addition, a number of frequently-accessed classes can be imported
   from the invokeai.model and invokeai.generator modules:

   from invokeai.generator import ( Generator, PipelineIntermediateState,
                                    StableDiffusionGeneratorPipeline, infill_methods)

   from invokeai.models import ( ModelManager, SDLegacyType
                                 InvokeAIDiffuserComponent, AttentionMapSaver,
                                 DDIMSampler, KSampler, PLMSSampler,
                                 PostprocessingSettings )

invokeai/backend/__init__.py

@@ -3,3 +3,4 @@ Initialization file for invokeai.backend
 '''
 from .invoke_ai_web_server import InvokeAIWebServer
 
+