refactor(parts): move yaml utils out of lifecycle

[mr-cal]

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Move 4 snapcraft.yaml utility functions out of lifecycle module into the yaml_utils module. No functional changes.

I've been wanting to do this for some time. Working on #4318 is an excuse to get it done since it requires importing these functions.

(CRAFT-1972)

[codecov-commenter]

Codecov Report

Merging #4338 (b2d7c7b) into main (9ad7a93) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #4338   +/-   ##
=======================================
  Coverage   89.11%   89.12%           
=======================================
  Files         300      300           
  Lines       20631    20635    +4     
=======================================
+ Hits        18386    18390    +4     
  Misses       2245     2245           

Files Changed	Coverage Δ
snapcraft/commands/discovery.py	91.48% <100.00%> (ø)
snapcraft/commands/extensions.py	100.00% <100.00%> (ø)
snapcraft/commands/init.py	100.00% <100.00%> (ø)
snapcraft/commands/lint.py	100.00% <100.00%> (ø)
snapcraft/commands/remote.py	79.48% <100.00%> (ø)
snapcraft/parts/lifecycle.py	91.82% <100.00%> (-1.09%)	⬇️
snapcraft/parts/yaml_utils.py	96.66% <100.00%> (+3.33%)	⬆️

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[cmatsuoka]

Ship it!