cmd/go: fix -coverpkg not ignoring special directories

The pattern passed to `-coverpkg` when running `go test` would not
ignore directories usually ignored by the `go` command, i.e. those
beginning with "." or "_" are ignored by the go tool, as are directories
named "testdata".

Fix this by adding an explicit check for these (by following a similar
check in `src/cmd/doc/dirs.go`[1]) allowing us to ignore them. The
scope of the change is limted to package matching to only the -coverpkg
flag of `go test` to avoid impacting -gcflags and the other per package
flags, e.g. we don't want to change behaviour for a user building
something that imports a leading dot package who wants to set gcflags
for it

See linked issue for a reproduction.

Fixes #66038

[1] https://go.googlesource.com/go/+/16e5d24480dca7ddcbdffb78a8ed5de3e5155dec/src/cmd/doc/dirs.go#136

src/cmd/go/internal/test/test.go

@@ -858,7 +858,10 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 	if cfg.BuildCoverPkg != nil {
 		match := make([]func(*load.Package) bool, len(cfg.BuildCoverPkg))
 		for i := range cfg.BuildCoverPkg {
-			match[i] = load.MatchPackage(cfg.BuildCoverPkg[i], base.Cwd())
+			match[i] = func(p *load.Package) bool {
+				cwd := base.Cwd()
+				return load.MatchPackage(cfg.BuildCoverPkg[i], cwd)(p) && !isUnderSpecial(cwd, p.Dir)
+			}
 		}
 
 		// Select for coverage all dependencies matching the -coverpkg
@@ -2112,3 +2115,36 @@ func testBinaryName(p *load.Package) string {
 
 	return elem + ".test"
 }
+
+// isUnderSpecial checks whether dir is contained within a 'special' directory under 'cwd'.
+// A directory is special if it beings with "." or "_" , or is called "testdata"
+func isUnderSpecial(cwd string, dir string) bool {
+	rel, err := filepath.Rel(cwd, dir)
+	if err != nil {
+		return false
+	}
+
+	hasAnyPrefix := func(dir string, prefixes ...string) bool {
+		for _, prefix := range prefixes {
+			if strings.HasPrefix(dir, prefix) {
+				return true
+			}
+		}
+		return false
+	}
+
+	sep := string(filepath.Separator)
+	if rel == "." || hasAnyPrefix(rel, ".."+sep) {
+		// Not a special directory under 'cwd', so can return immediately
+		return false
+	}
+
+	// Otherwise avoid special directories "testdata" or beginning with ".", "_".
+	pathComponents := strings.Split(rel, sep)
+	for _, elem := range pathComponents {
+		if hasAnyPrefix(elem, ".", "_") || elem == "testdata" {
+			return true
+		}
+	}
+	return false
+}

src/cmd/go/testdata/script/coverpkg_ignores_special_dirs.txt

@@ -0,0 +1,57 @@
+# test for https://github.com/golang/go/issues/66038
+[short] skip
+
+env GO111MODULE=off
+# files
+env GOPATH=$WORK/.dir${:}$WORK/_dir${:}$WORK/testdata
+
+cd $WORK
+go test -coverpkg=./... ./...
+stdout 'coverage: 100.0%'
+
+-- $WORK/a.go --
+package a
+
+// trivial function with 100% test coverage
+
+import (
+	_ "dot_dir"
+	_ "under_dir"
+	_ "testdata_dir"
+)
+
+func F(i int) int {
+	return i*i
+}
+
+-- $WORK/a_test.go --
+package a
+
+import (
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	F(2)
+}
+
+-- $WORK/.dir/src/dot_dir/b.go --
+package dot_dir
+
+func G(i int) int {
+	return i*i
+}
+
+-- $WORK/_dir/src/under_dir/b.go --
+package dot_dir
+
+func G(i int) int {
+	return i*i
+}
+
+-- $WORK/testdata/src/testdata_dir/b.go --
+package testdata_dir
+
+func G(i int) int {
+	return i*i
+}