golangci: add import alias linter

[corverroos]

Adds imports alias linters.

category: misc
ticket: #1684

[codecov]

Codecov Report

Base: 54.27% // Head: 54.53% // Increases project coverage by +0.25% 🎉

Coverage data is based on head (bc1b7dc) compared to base (bfff703).
Patch coverage: 53.65% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1696      +/-   ##
==========================================
+ Coverage   54.27%   54.53%   +0.25%     
==========================================
  Files         159      159              
  Lines       20620    20620              
==========================================
+ Hits        11192    11245      +53     
+ Misses       7930     7880      -50     
+ Partials     1498     1495       -3     

Impacted Files	Coverage Δ
core/interfaces.go	0.00% <ø> (ø)
testutil/beaconmock/beaconmock.go	46.03% <0.00%> (ø)
testutil/beaconmock/options.go	33.50% <0.00%> (ø)
testutil/validatormock/validatormock.go	30.81% <47.05%> (ø)
core/unsigneddata.go	47.34% <47.61%> (ø)
core/signeddata.go	44.42% <55.31%> (ø)
core/fetcher/fetcher.go	62.85% <66.66%> (ø)
core/validatorapi/router.go	65.07% <70.58%> (ø)
core/dutydb/memory.go	73.56% <100.00%> (ø)
core/validatorapi/validatorapi.go	58.76% <100.00%> (ø)
... and 4 more

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