Miscellaneous code cleanup procedures

[jgbradley1]

Description

This PR contains several miscellaneous code cleanup changes:

• move graphrag.index.storage -> graphrag.storage
• removed multiple emitter options - graphrag will only support exporting files in the parquet format when storage.type=file
• updated some of the terminology used in the code/docs/comments from emitter to exporter for better clarity. Open to reverting back to the old terminology if there are strong opinions against exporter.
• renamed all factory related files to factory.py for better consistency across the codebase
• refactored code under graphrag.index.utils to remove unnecessary "shortcut" functions that were simply calling factory methods. Other parts of the library should call factory methods directly to reduce complexity and increase the amount of shared code.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

[jgbradley1]

For better alignment, graphrag.index.cache could be moved up a level to graphrag.cache as well for a flatter folder structure. I could do that in another PR or this one if we want to move it around.

[jgbradley1]

I went ahead and moved graphrag.index.cache up a level to graphrag.cache to be in better alignment with the design we're shooting for.