Use the new logging interface throughout

[lhchavez]

No more explicit references to log15! #minor

[codecov]

Codecov Report

Merging #47 (0154a16) into main (878f919) will decrease coverage by 3.49%.
The diff coverage is 19.06%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   44.37%   40.88%   -3.50%     
==========================================
  Files          25       25              
  Lines        5832     6680     +848     
==========================================
+ Hits         2588     2731     +143     
- Misses       2985     3689     +704     
- Partials      259      260       +1     

Impacted Files	Coverage Δ
cmd/omegaup-grader/main.go	2.09% <0.00%> (-0.11%)	⬇️
cmd/omegaup-grader/metrics.go	0.00% <0.00%> (ø)
common/literalinput.go	27.76% <ø> (ø)
common/problemsettings.go	55.26% <ø> (ø)
common/run.go	5.35% <ø> (ø)
runner/benchmark.go	0.00% <0.00%> (ø)
runner/ci/ci.go	44.83% <0.00%> (-1.64%)	⬇️
runner/noop_sandbox.go	0.00% <0.00%> (ø)
runner/validator.go	87.34% <ø> (ø)
cmd/omegaup-grader/frontend_handler.go	12.73% <2.51%> (-3.79%)	⬇️
... and 10 more

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