Rust: Fix bad join in getAPrivateVisibleModule

Before
```
Pipeline standard for PathResolution::getAPrivateVisibleModule/1#3829a5ee@822d5hwq was evaluated in 24 iterations totaling 16ms (delta sizes total: 4843).
         105047   ~63652%    {2} r1 = SCAN `PathResolution::resolvePathPrivate/3#56db2cdf#reorder_1_2_0_3#prev_delta` OUTPUT In.0, In.0

             69       ~0%    {2} r2 = JOIN `#PathResolution::ItemNode.getImmediateParentModule/0#dispred#57c4c6d5Plus#bf#reorder_1_0#prev_delta` WITH `PathResolution::resolvePathPrivate/3#56db2cdf#reorder_1_2_0_3#prev` ON FIRST 1 OUTPUT Lhs.0, Lhs.1

        5766690  ~148309%    {2} r3 = JOIN `PathResolution::resolvePathPrivate/3#56db2cdf#reorder_1_2_0_3#prev_delta` WITH `#PathResolution::ItemNode.getImmediateParentModule/0#dispred#57c4c6d5Plus#bf#reorder_1_0#prev` ON FIRST 1 OUTPUT Lhs.0, Rhs.1

        5871806  ~143984%    {2} r4 = r1 UNION r2 UNION r3
           6859     ~148%    {2}    | AND NOT `PathResolution::getAPrivateVisibleModule/1#3829a5ee#prev`(FIRST 2)
                             return r4
```

After
```
Pipeline standard for PathResolution::getAPrivateVisibleModule/1#3829a5ee@5edefhwp was evaluated in 12 iterations totaling 0ms (delta sizes total: 3515).
         339   ~1%    {2} r1 = SCAN `PathResolution::isItemParent/1#d5e587d6#prev_delta` OUTPUT In.0, In.0

        3130   ~0%    {2} r2 = JOIN `PathResolution::isItemParent/1#d5e587d6#prev_delta` WITH `#PathResolution::ItemNode.getImmediateParentModule/0#dispred#57c4c6d5Plus#bf#reorder_1_0#prev` ON FIRST 1 OUTPUT Lhs.0, Rhs.1

          46   ~0%    {2} r3 = JOIN `#PathResolution::ItemNode.getImmediateParentModule/0#dispred#57c4c6d5Plus#bf#reorder_1_0#prev_delta` WITH `PathResolution::isItemParent/1#d5e587d6#prev` ON FIRST 1 OUTPUT Lhs.0, Lhs.1

        3515   ~2%    {2} r4 = r1 UNION r2 UNION r3
        3515   ~2%    {2}    | AND NOT `PathResolution::getAPrivateVisibleModule/1#3829a5ee#prev`(FIRST 2)
                      return r4
```

Author: hvitved

rust/ql/lib/codeql/rust/internal/PathResolution.qll

@@ -981,6 +981,11 @@ private ItemNode resolvePathPrivate(
   )
 }
 
+pragma[nomagic]
+private predicate isItemParent(ModuleLikeNode itemParent) {
+  exists(resolvePathPrivate(_, itemParent, _))
+}
+
 /**
  * Gets a module that has access to private items defined inside `itemParent`.
  *
@@ -991,7 +996,7 @@ private ItemNode resolvePathPrivate(
  */
 pragma[nomagic]
 private ModuleLikeNode getAPrivateVisibleModule(ModuleLikeNode itemParent) {
-  exists(resolvePathPrivate(_, itemParent, _)) and
+  isItemParent(itemParent) and
   result.getImmediateParentModule*() = itemParent
 }