fix(golang): skip remote license lookup for standard library module paths (#5192)

With search-remote-licenses enabled, every module name went to the proxy,
including toolchain binaries whose main module is synthesized from the
package path, such as cmd/cgo. The proxy answers 404, and the direct
fallback then treats the path as a repository host, producing requests
like https://cmd/cgo/info/refs?service=git-upload-pack.

Reuse isStandardImportPath, already in this package, to skip the remote
search for paths whose first element carries no dot. Those are never
publishable module paths, so neither a proxy nor a repository can resolve
them.

Fixes #3149

Signed-off-by: Luan Taraschi <130802253+luantaraschi@users.noreply.github.com>

syft/pkg/cataloger/golang/licenses.go

@@ -111,8 +111,9 @@ func (c *goLicenseResolver) getLicenses(ctx context.Context, resolver file.Resol
 		}
 	}
 
-	// download from remote sources
+	// download from remote sources; standard library and toolchain paths are not publishable
-	if c.opts.SearchRemoteLicenses {
+	// module paths, so neither a proxy nor a repository has anything to resolve for them
+	if c.opts.SearchRemoteLicenses && !isStandardImportPath(moduleName) {
 		pkgLicenses, err = c.getLicensesFromRemote(ctx, moduleName, moduleVersion)
 		if err != nil {
 			log.WithFields("error", err, "module", moduleName, "version", moduleVersion).Debug("unable to read golang licenses remote")

syft/pkg/cataloger/golang/licenses_test.go

@@ -12,6 +12,7 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
+	"sync/atomic"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -235,6 +236,35 @@ func Test_LicenseSearch(t *testing.T) {
 	}
 }
 
+func Test_remoteLicenseSearchSkipsStandardLibrary(t *testing.T) {
+	ctx := pkgtest.Context(t)
+
+	var requests atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		requests.Add(1)
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer server.Close()
+
+	// module paths whose first element carries no dot are never publishable, so there is
+	// nothing for a proxy or a repository to resolve
+	for _, moduleName := range []string{"cmd/cgo", "std", "runtime", "internal/abi", "command-line-arguments"} {
+		t.Run(moduleName, func(t *testing.T) {
+			requests.Store(0)
+
+			l := newGoLicenseResolver("", CatalogerConfig{
+				SearchRemoteLicenses: true,
+				Proxies:              []string{server.URL},
+			})
+
+			lics := l.getLicenses(ctx, fileresolver.Empty{}, moduleName, "(devel)")
+
+			require.Empty(t, lics)
+			require.Zero(t, requests.Load(), "expected no remote lookup for a standard library module path")
+		})
+	}
+}
+
 func Test_processCaps(t *testing.T) {
 	tests := []struct {
 		name     string