See https://github.com/elves/elvish/pull/1190 for details.
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Coverage of Go language (golang) files implicitly excludes any line not in the coverage data.
The Codecov analysis includes, as not executed, statements in
import blocks as well as other constructs that are implicitly executed but not included in the coverage data. This noticably decreases the coverage metric for individual Go (golang) files as well as, obviously, the coverage percentage for the project as a whole.