Move LLM-related components into LLM module (#74)

* Move splitter into indexing module
* Rename post_processing module to parsers
* Migrate LLM-specific composite pipelines into llms module

This change moves the `splitters` module into `indexing` module. The `indexing` module will be created soon, to house `indexing`-related components.

This change renames `post_processing` module into `parsers` module. Post-processing is a generic term which provides very little information. In the future, we will add other extractors into the `parser` module, like Metadata extractor...

This change migrates the composite elements into `llms` module. These elements heavily assume that the internal nodes are llm-specific. As a result, migrating these elements into `llms` module will make them more discoverable, and simplify code base structure.

tests/test_composite.py

@@ -3,15 +3,15 @@ from copy import deepcopy
 import pytest
 from openai.types.chat.chat_completion import ChatCompletion
 
-from kotaemon.composite import (
+from kotaemon.llms import (
+    AzureChatOpenAI,
+    BasePromptComponent,
     GatedBranchingPipeline,
     GatedLinearPipeline,
     SimpleBranchingPipeline,
     SimpleLinearPipeline,
 )
-from kotaemon.llms import BasePromptComponent
-from kotaemon.llms.chats.openai import AzureChatOpenAI
-from kotaemon.post_processing.extractor import RegexExtractor
+from kotaemon.parsers import RegexExtractor
 
 _openai_chat_completion_response = ChatCompletion.parse_obj(
     {