api: reorganize artifact generation API

[kyrofa]

This PR continues the reorganization of #188 by extracting the artifact generation API into its own module.

Note that this introduces no behavioral changes, it's just moving code around.

[kyrofa]

Note that using lists as default values should probably be fixed in a follow-up.

[kyrofa]

@mikaelarguedas you'll notice I'm not using any git tricks on this one, because I don't know any that work when moving code into a file that already exists. Do you?

[codecov]

Codecov Report

Merging #195 into master will decrease coverage by 0.41%.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   55.38%   54.96%   -0.42%     
==========================================
  Files          17       18       +1     
  Lines         585      584       -1     
  Branches       52       52              
==========================================
- Hits          324      321       -3     
- Misses        247      249       +2     
  Partials       14       14              

Flag	Coverage Δ
#unittests	54.96% <9.09%> (-0.42%)	⬇️

Impacted Files	Coverage Δ
sros2/sros2/api/__init__.py	55.00% <ø> (+22.30%)	⬆️
sros2/sros2/api/_artifact_generation.py	0.00% <0.00%> (ø)
sros2/sros2/verb/generate_artifacts.py	0.00% <0.00%> (ø)
sros2/sros2/api/_utilities.py	86.11% <42.85%> (-4.66%)	⬇️

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[mikaelarguedas]

you'll notice I'm not using any git tricks on this one, because I don't know any that work when moving code into a file that already exists. Do you?

That's where my git-fu stops as well. Git is alledgedly able to track such copies using git blame -C but we won't be able to track this through the github blame interface. In that case we'll just have to live with following the change by hand. It's still moved in the same commit so not that hard to track

[kyrofa]

Alright this should be ready for another pass.

[mikaelarguedas]

looks great!

[artivis]

LGTM.