Answer architecture round 9: each data package exports Modules, a locale's importing its country's geo package as a bench chose, and a namespace mounts a tree through an fs.FS wrapper, so internal/datafiles walks file-system folders alone

data/misc/fs.go

@@ -1,9 +1,15 @@
 // Package misc is fejkdata's locale-neutral module, whose categories are misc.*.
 package misc
 
-import "embed"
+import (
+	"embed"
+	"io/fs"
+)
 
-// FS holds the module's categories and its manifest; pass it to fejkdata.WithDataFS.
+// FS holds the module's categories and its manifest.
 //
 //go:embed *.json *.tsv .fejkdata.json
 var FS embed.FS
+
+// Modules returns FS, which reads no other module, to pass to fejkdata.WithDataFS.
+func Modules() []fs.FS { return []fs.FS{FS} }