cmd/go: exclude empty cover package

[forsaken628]

Fixes #43242

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Message from Daniel Martí:

Patch Set 1: Code-Review-1

(1 comment)

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[forsaken628]

Fixes #27333

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Message from Michael Zhou:

Patch Set 2:

(1 comment)

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[mvdan]

The issue got closed as a duplicate, and I've just sent a fix for the original issue with a test. Let's focus there. Thanks!