overlord: add support for graphviz dot representation of change graphs

[pedronis]

this adds a package overlord/state/dot for this

it can be used to create canonical repr for the task dep graph of changes, that could be used in tests

it also can display the graphs for debugging

[codecov-commenter]

Codecov Report

Merging #13275 (d2113e3) into master (73c90a1) will decrease coverage by 0.03%. Report is 35 commits behind head on master. The diff coverage is 67.53%.

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@@            Coverage Diff             @@
##           master   #13275      +/-   ##
==========================================
- Coverage   78.83%   78.81%   -0.03%     
==========================================
  Files        1016     1014       -2     
  Lines      126663   126419     -244     
==========================================
- Hits        99857    99632     -225     
+ Misses      20557    20553       -4     
+ Partials     6249     6234      -15     

Flag	Coverage Δ
unittests	78.81% <67.53%> (-0.03%)	:arrow_down:

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Files	Coverage Δ
interfaces/builtin/gpio_control.go	100.00% <ø> (ø)
overlord/restart/restart.go	83.88% <100.00%> (+0.60%)	:arrow_up:
secboot/luks2/cryptsetup.go	72.05% <94.44%> (-1.40%)	:arrow_down:
overlord/devicestate/devicemgr.go	76.83% <76.92%> (-0.05%)	:arrow_down:
overlord/devicestate/handlers_serial.go	74.73% <52.94%> (-1.19%)	:arrow_down:
overlord/overlord.go	75.10% <0.00%> (-5.31%)	:arrow_down:
overlord/state/dot/dot.go	77.21% <77.21%> (ø)

... and 25 files with indirect coverage changes

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