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/en_US/fs.go

@@ -1,9 +1,19 @@
-// Package enus is fejkdata's en_US locale module, whose categories are en_US.*. Its address reads geous's places.
+// Package enus is fejkdata's en_US locale module, whose categories are en_US.*; its address reads
+// geo.US, which package geous holds.
 package enus
 
-import "embed"
+import (
+	"embed"
+	"io/fs"
 
-// FS holds the module's categories and its manifest; pass it to fejkdata.WithDataFS.
+	geous "github.com/larvit/fejkdata/data/geo/US"
+)
+
+// FS holds the locale alone, its categories and its manifest; pass Modules unless your own
+// data stands in for geo.US.
 //
 //go:embed *.json *.tsv .fejkdata.json
 var FS embed.FS
+
+// Modules returns FS and every module it reads, to pass to fejkdata.WithDataFS.
+func Modules() []fs.FS { return append([]fs.FS{FS}, geous.Modules()...) }

data/geo/SE/fs.go

@@ -1,9 +1,15 @@
 // Package geose is fejkdata's Swedish places module, whose categories are geo.SE.*.
 package geose
 
-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} }

data/geo/US/fs.go

@@ -1,9 +1,15 @@
 // Package geous is fejkdata's US places module, whose categories are geo.US.*.
 package geous
 
-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} }

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} }

data/sv_SE/fs.go

@@ -1,9 +1,19 @@
-// Package svse is fejkdata's sv_SE locale module, whose categories are sv_SE.*. Its address reads geose's places.
+// Package svse is fejkdata's sv_SE locale module, whose categories are sv_SE.*; its address reads
+// geo.SE, which package geose holds.
 package svse
 
-import "embed"
+import (
+	"embed"
+	"io/fs"
 
-// FS holds the module's categories and its manifest; pass it to fejkdata.WithDataFS.
+	geose "github.com/larvit/fejkdata/data/geo/SE"
+)
+
+// FS holds the locale alone, its categories and its manifest; pass Modules unless your own
+// data stands in for geo.SE.
 //
 //go:embed *.json *.tsv .fejkdata.json
 var FS embed.FS
+
+// Modules returns FS and every module it reads, to pass to fejkdata.WithDataFS.
+func Modules() []fs.FS { return append([]fs.FS{FS}, geose.Modules()...) }