save modidx submodule path in bytecode form

This change should have been part of 9ba663aa77.

original commit: f099eec2af

collects/compiler/decompile.rkt

@@ -161,7 +161,8 @@
 (define (mpi->string modidx)
   (cond
    [(symbol? modidx) modidx]
-   [else (collapse-module-path-index modidx (current-directory))]))
+   [else 
+    (collapse-module-path-index modidx (current-directory))]))
 
 (define (decompile-module mod-form orig-stack stx-ht mod-name)
   (match mod-form

collects/compiler/zo-marshal.rkt

@@ -895,7 +895,9 @@
         (out-byte CPT_MODULE_INDEX out)
         (let-values ([(name base) (module-path-index-split v)])
           (out-anything name out)
-          (out-anything base out))]
+          (out-anything base out)
+          (unless (or name base)
+            (out-anything (module-path-index-submodule v) out)))]
        [(stx encoded)
         (out-byte CPT_STX out)
         (out-anything encoded out)]

collects/compiler/zo-parse.rkt

@@ -840,7 +840,12 @@
                        (eq? cpt-tag 'let-one-unused))]
         [(branch)
          (make-branch (read-compact cp) (read-compact cp) (read-compact cp))]
-        [(module-index) (module-path-index-join (read-compact cp) (read-compact cp))]
+        [(module-index) 
+         (define name (read-compact cp))
+         (define base (read-compact cp))
+         (if (or name base)
+             (module-path-index-join name base)
+             (module-path-index-join #f #f (read-compact cp)))]
         [(module-var)
          (let ([mod (read-compact cp)]
                [var (read-compact cp)]