Broaden megatron-core packages.find to megatron* (namespaces), keep plugins

[Shi-Dong]

Summary

Builds on the merged #49 (Package miles_megatron_plugins with Megatron Core) by broadening [tool.setuptools.packages.find].include from megatron.core* to megatron* with namespaces = true, while keeping miles_megatron_plugins in the include list.

This lets callers (e.g. the miles training runner) pip install -e . this checkout and import from megatron.training, megatron.legacy, megatron.rl, etc. without a PYTHONPATH override — megatron.training.initialize imports from megatron.legacy at module load, so a narrower include list breaks transitively. namespaces = true is required because megatron, megatron.legacy, and several sub-packages are PEP-420 namespace packages (no __init__.py).

Why

The miles submoduling stack pins this repo as third_party/Megatron-LM and bundles both megatron.* and miles_megatron_plugins into the miles-rl wheel directly from the submodule. That requires the source checkout to expose the full megatron* tree (not just megatron.core) and to keep packaging the plugins. This PR reconciles both needs in one place, on top of the current miles-main (post-#49).

Supersedes

Replaces the obsolete pair:

• Remove miles_megatron_plugins/ (moved to radixark/miles) #32 (Remove miles_megatron_plugins/) — contradicts Package miles_megatron_plugins with Megatron Core #49; we keep the plugins inside Megatron-LM.
• Broaden packages.find.include to expose megatron.training #33 (Broaden packages.find.include) — folded into this single commit on top of the current miles-main.

Test plan

• pip install -e . on this checkout, then python -c "import megatron.training, megatron.legacy, miles_megatron_plugins" with no PYTHONPATH override
• Build the miles-rl wheel from the miles submoduling stack and confirm megatron* + miles_megatron_plugins land at the top level of site-packages