cmd/go: should include packages without tests in cover profile file

[13rac1]

What version of Go are you using (go version)?

go version go1.11 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOOS="linux"

What did you do?

Created a new project with two sub-packages:

.
├── notests
│   └── notests.go
└── tests
    ├── tests.go
    └── tests_test.go

2 directories, 3 files

notests/notests.go

package notests

import "fmt"

func untested() {
	fmt.Print("untested")
}

tests/tests.go

package tests

import "fmt"

func tested() {
	fmt.Print("tested")
}

tests/tests_test.go

package tests

import "testing"

func TestTested(t *testing.T) {
	tested()
}

Ran go test ./... -v -cover -coverprofile cover.out

What did you expect to see?

$ cat cover.out
mode: set
empty-cover-profile/notests/notests.go:7.17,9.2 1 0
empty-cover-profile/tests/tests.go:7.15,9.2 1 1

Coverage details for both files.

#18909 requested this functionality, but it wasn't implemented in https://golang.org/cl/76875.

What did you see instead?

$ cat cover.out
mode: set
empty-cover-profile/tests/tests.go:7.15,9.2 1 1

Coverage details only for the file with a test in the same directory.

Workaround

echo "package notests" > notests/notests_test.go

Creating an empty test file creates coverage for both files.

[13rac1]

Dupe of #24570