Fix architecture subrepos

src/core/package.go

@@ -67,7 +67,6 @@ func (pkg *Package) TargetOrDie(name string) *BuildTarget {
 	return t
 }
 
-
 // AddTarget adds a new target to this package with the given name.
 // It doesn't check for duplicates.
 func (pkg *Package) AddTarget(target *BuildTarget) {

src/core/state.go

@@ -255,8 +255,7 @@ func (state *BuildState) Initialise(subrepo *Subrepo) (err error) {
 		// handled for us already in plz.go
 		if state.CurrentSubrepo != "" {
 			state.RepoConfig = &Configuration{}
-			err = readSubrepoConfig(state.RepoConfig, subrepo)
-			if err != nil {
+			if err := readSubrepoConfig(state.RepoConfig, subrepo); err != nil {
 				return
 			}
 			if err = validateSubrepoNameAndPluginConfig(state.Config, state.RepoConfig, subrepo); err != nil {

src/core/subrepo.go

@@ -36,10 +36,15 @@ type Subrepo struct {
 }
 
 func NewSubrepo(state *BuildState, name, root string, target *BuildTarget, arch cli.Arch, isCrosscompile bool) *Subrepo {
+	subrepoFS := os.DirFS(root)
+	if root == "" {
+		// This happens for architecture subrepos, which should use the same FS as the host repo
+		subrepoFS = fs.HostFS
+	}
 	return &Subrepo{
 		Name:           name,
 		Root:           root,
-		FS:             os.DirFS(root),
+		FS:             subrepoFS,
 		State:          state,
 		Target:         target,
 		Arch:           arch,