refactor(domain): use strings.CutPrefix

[favonia]

Close #236.

• cmd/go: go test -cover & go test -coverprofile should always output a coverage golang/go#24570

[codecov]

Codecov Report

Merging #383 (692f36d) into main (1519e20) will increase coverage by 9.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
+ Coverage   87.89%   97.40%   +9.50%     
==========================================
  Files          32       30       -2     
  Lines        2049     1850     -199     
==========================================
+ Hits         1801     1802       +1     
+ Misses        236       36     -200     
  Partials       12       12              

Flag	Coverage Δ
unittests	97.40% <100.00%> (+9.50%)	⬆️

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Impacted Files	Coverage Δ
internal/domainexp/parser.go	100.00% <ø> (ø)
internal/domain/domain.go	100.00% <100.00%> (ø)
cmd/ddns/ddns.go
internal/droproot/drop.go

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[favonia]

Everything works except that the coverage data generated by Go 1.20 are off: packages without test files do not appear in the coverage report even with -coverpkg.