Improve CLI speed with lazy imports

[jgbradley1]

Description

This is an optional PR to consider. Because of too many imports getting executed in various __init__.py files throughout the library, the CLI is slow to start up. The initialization of so many imports also makes the initial import of graphrag slow as well.

Proposed Changes

Following some tips from this article, I profiled the startup time for the entire library with a focus on improving the CLI startup time. Some import statements involving very large libraries or long-processing times have been moved inside the function calls that rely on them.

Across the entire library, nearly all __init__.py files are empty now.

The following screenshot shows the startup time for the CLI code when the package is first installed.

After the library has been loaded once (python generates bytecode to speed up future imports), we can achieve an even faster load time of 0.3 seconds.

Tab completion with the CLI is also now very responsive.

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).

Additional Notes

This PR closes #1299 as well. Additional comments were added in places where import statements may appear unusual at first explaining why they have been moved there.

A small patch for Typer path completion was added to the CLI to improve the CLI experience.