w6c+selfhost: same-module preference for bare-leaf lookup

cmd/wcc/ww.h

@@ -509,6 +509,7 @@ Sym	*scope_define_in_module(Scope*, const char *name, const char *mod,
 Sym	*scope_lookup(Scope*, const char *name);	/* walk up parents */
 Sym	*scope_lookup_local(Scope*, const char *name);
 Sym	*scope_lookup_in_module(Scope*, const char *mod, const char *name);
+Sym	*scope_lookup_prefer(Scope*, const char *mod, const char *name);
 
 /* ---- checker (check.c) -------------------------------------------- */
 typedef struct Checker Checker;
@@ -517,6 +518,12 @@ struct Checker {
 	Scope	*top;	/* file scope */
 	Scope	*cur;	/* current scope */
 	Type	*ret;	/* expected return type of current fn (or NULL) */
+	const char *cur_mod;	/* importing-module bareword for the decl
+				 * currently being checked; NULL for primary
+				 * compilation unit. Drives same-module
+				 * preference in bare-leaf lookups so a bare
+				 * `read` inside lib/os resolves to os.read
+				 * rather than colliding io.read. */
 	int	loops;	/* nesting count for break/continue */
 	int	errs;
 };