Make the dummy dep on licenses dir stronger

Without an actual code-level dependency, `go mod vendor` will elide the
licenses directory, which is where the actual DB files are stored.
Without those, it's not very useful to vendor this lib.

file_system_resources.go

@@ -21,8 +21,14 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+
+	"github.com/google/licenseclassifier/licenses"
 )
 
+// forceDepOnLicenses is used to force a package dependency on the licenses
+// dir, which includes the LicenseArchive and ForbiddenLicenseArchive files.
+type forceDepOnLicenses licenses.Dummy
+
 const (
 	// LicenseDirectory is the directory where the prototype licenses are kept.
 	LicenseDirectory = "src/github.com/google/licenseclassifier/licenses"

licenses/dummy.go

@@ -1,5 +1,7 @@
-// +build tools
-
+// Package dummy is a placeholder. This allows consumers of licenseclassifier
+// to use Go modules and vendoring and still find in the licenses.db file.
 package licenses
 
-// Placeholder, allows others to pull in the licenses.db file via go.mod.
+// Dummy is a pointless type which is used to satisfy Go's dependency-tracking
+// to include this directory.
+type Dummy struct{}