ui: Only suggest modules that export the unbound variable identifier.

Fixes <https://bugs.gnu.org/43498>.
Reported by Tobias Geerinckx-Rice <me@tobias.gr>.

* guix/ui.scm (known-variable-definition): Check for variables in the
public interface of HEAD, not in HEAD itself.
* tests/guix-build.sh: Add test.

guix/ui.scm

@@ -297,7 +297,8 @@ VARIABLE and return it, or #f if none was found."
                                   (hash-map->list (lambda (name module)
                                                     module)
                                                   (module-submodules head)))))
-             (match (module-local-variable head variable)
+             (match (and=> (module-public-interface head)
+                           (cut module-local-variable <> variable))
                (#f (loop next suggestions visited))
                (_
                 (match (module-name head)

tests/guix-build.sh

@@ -198,6 +198,33 @@ grep "forget.*(guix build-system gnu)" "$module_dir/err" # hint
 
 rm -f "$module_dir"/*
 
+# Unbound variable: don't suggest modules that do not export the variable.
+cat > "$module_dir/aa-private.scm" <<EOF
+(define-module (aa-private))
+(define make-thing #f)
+(set! make-thing make-thing)   ;don't inline
+EOF
+
+cat > "$module_dir/bb-public.scm" <<EOF
+(define-module (bb-public) #:export (make-thing))
+(define make-thing identity)
+EOF
+
+cat > "$module_dir/cc-user.scm" <<EOF
+;; Make those module available in the global name space.
+(load-from-path "aa-private.scm")
+(load-from-path "bb-public.scm")
+
+(define-module (cc-user))
+(make-thing 42)
+EOF
+! guix build -f "$module_dir/cc-user.scm" -n 2> "$module_dir/err"
+cat "$module_dir/err"
+grep "make-thing.*unbound" "$module_dir/err"		 # actual error
+grep "forget.*(bb-public)" "$module_dir/err"		 # hint
+
+rm -f "$module_dir"/*
+
 # Wrong 'define-module' clause reported by 'warn-about-load-error'.
 cat > "$module_dir/foo.scm" <<EOF
 (define-module (something foo)