Repository: go
Issue: cmd/go: coverpkg doesn't ignore directories starting with '.'
Labels: NeedsInvestigation,GoCommand

### Go version

go version go1.22.0 linux/amd64

### Output of `go env` in your module/workspace:

```shell
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/mjh/.cache/go-build'
GOENV='/home/mjh/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/mjh/.local/share/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/mjh/.local/share/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/mjh/sdk/go1.22.0'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/mjh/sdk/go1.22.0/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/mjh/src/personal/go-cov/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3982892531=/tmp/go-build -gno-record-gcc-switches'
```


### What did you do?

Ran Go tests with `-coverpkg=./...` in a directory which included `GOMODCACHE/GOCACHE` in a subdirectory starting with `"."` (the use case is for caching in GitLab, which requires directories stored in caches exist in the working directory, see their Go example https://docs.gitlab.com/ee/ci/caching/#cache-go-dependencies)

Here's a bash script to reproduce:

```bash
#!/usr/bin/env bash

set -o errexit -o pipefail -o nounset

mkdir proj
cd proj
go mod init example-proj

# create a trivial lib with 100% test coverage
cat <<EOF > lib.go
package lib

import (
	// add an unused dependency, just so something is stored in the cache directory
	_ "golang.org/x/tools/cover"
)

func Sum(a int, b int) int { 
	return a + b
}
EOF

cat <<EOF > lib_test.go
package lib

import (
	"testing"
)

func TestSum(t *testing.T) {
	if Sum(1, 2) != 3 {
		t.Error("fail")
	}
}
EOF

# .go contains directories we wish to cache
mkdir .go
export GOMODCACHE="$PWD/.go/mod_cache" 
export GOCACHE="$PWD/.go/cache"

go mod tidy
go test -coverprofile=coverage.out -coverpkg=./... ./...
```

### What did you see happen?

The modules under `.go` were included in test coverage, output of the above script (note the coverage % in the last line):    

```
go: creating new go.mod: module example-proj
go: finding module for package golang.org/x/tools/cover
go: downloading golang.org/x/tools v0.18.0
go: found golang.org/x/tools/cover in golang.org/x/tools v0.18.0
ok  	example-proj	0.002s	coverage: 0.8% of statements in ./...
```

inspecting the coverage profile:

```console
$ grep --invert-match "$(go list -m)" coverage.out | head
mode: set
golang.org/x/tools/cover/profile.go:37.41,37.58 1 0
golang.org/x/tools/cover/profile.go:38.41,38.81 1 0
golang.org/x/tools/cover/profile.go:39.41,39.68 1 0
golang.org/x/tools/cover/profile.go:43.57,45.16 2 0
golang.org/x/tools/cover/profile.go:45.16,47.3 1 0
golang.org/x/tools/cover/profile.go:48.2,49.36 2 0
golang.org/x/tools/cover/profile.go:54.64,62.15 4 0
golang.org/x/tools/cover/profile.go:62.15,64.17 2 0
golang.org/x/tools/cover/profile.go:64.17,66.48 2 0
```

### What did you expect to see?

Modules under the `.go` directory to be ignored, per https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns

> Directory and file names that begin with "." or "_" are ignored by the go tool, as are directories named "testdata". 

EDIT: for anyone looking for a workaround, I just filtered out lines not belonging to my module in the coverage file:

    awk -v go_mod="$(go list -m)" 'NR==1 || $0 ~ "^"go_mod' "coverage.in" > coverage.out