Remove legacy uppercase MaxText from src, to ensure maxtext works on MacOS which has a case-insensitive filesystem.

[syrkis]

Description

This PR makes maxtext work on MacOS, where is currently broken. The removes legacy uppercase MaxText (which was being deprecated anyway) from src/, to ensure maxtext works on MacOS which has a case-insensitive filesystem. Without this, maxtext is broken on MacOS. Additionally, src/MaxText/__init__.py is moved to src/maxtext/__init__.py, and JAX was added as a dependency directly in pyproject.toml. Also basic guarding is added to not try to install CUDA, etc on Mac.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

FIXES: #3441

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Tests

I've tested by adding my fork as a dependency to a project on my Linux cluster.

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.