plugins: kernel: split kernel plugin into kernel and initrd plugins

[kubiko]

kernel plugin builds two components, kernel and initrd
While initrd is dependent on the kernel, it is a separate component.
Separating those 2 steps into 2 own plugins will allow greater reuse of the
kernel plugin as well as each plugin being owned by respective teams.

Initrd steps will always depend on the kernel step, but with
the plugin separation, prebuilt kernel binaries can be used with the initrd plugin
In the future inird might even become the official way to build kernel snap
from kernel deb packages.
For simplicity now ignoring signed kernels...

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[codecov-commenter]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (89961cf) 89.18% compared to head (0b302f5) 89.16%.

Files	Patch %	Lines
snapcraft_legacy/plugins/v2/_initrd_build.py	70.68%	34 Missing ⚠️
snapcraft_legacy/plugins/v2/initrd.py	90.56%	5 Missing ⚠️
snapcraft_legacy/plugins/v2/_kernel_build.py	93.75%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4302      +/-   ##
==========================================
- Coverage   89.18%   89.16%   -0.02%     
==========================================
  Files         321      324       +3     
  Lines       21777    21767      -10     
==========================================
- Hits        19421    19408      -13     
- Misses       2356     2359       +3     

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[sergiusens]

Ondra, this is fantastic, thanks for this.

I'll circle back to the kernel, foundations and core folks to get this reviewed as well