For consistency, rename new kernel plugin file to "kernel_plugin.py"

[rpjday]

The legacy plugin files were left as is to minimize churn.

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• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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[codecov-commenter]

Codecov Report

Merging #4164 (439e58a) into main (f0f4373) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #4164   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files         569      569           
  Lines       43744    43746    +2     
=======================================
+ Hits        41076    41078    +2     
  Misses       2668     2668           

Impacted Files	Coverage Δ
snapcraft/elf/elf_utils.py	95.38% <ø> (ø)
snapcraft/parts/plugins/kernel_plugin.py	100.00% <ø> (ø)
tests/unit/elf/test_elf_utils.py	100.00% <ø> (ø)
snapcraft/parts/plugins/__init__.py	100.00% <100.00%> (ø)
snapcraft/parts/plugins/register.py	100.00% <100.00%> (ø)
snapcraft/providers.py	97.75% <100.00%> (+0.02%)	⬆️
tests/unit/parts/plugins/test_kernel.py	98.57% <100.00%> (ø)
tests/unit/parts/test_lifecycle.py	100.00% <100.00%> (ø)
tests/unit/test_providers.py	100.00% <100.00%> (ø)

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