Shared: Fix join in FileSystem.qll

Before
```
Evaluated relational algebra for predicate FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d@d2a7eeoa on iteration 2 running pipeline standard with tuple counts:
         120   ~2%    {4} r1 = SCAN `FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d#prev_delta` OUTPUT In.1, In.0, In.2, In.3

           0   ~0%    {6} r2 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs#1` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Lhs.1, Lhs.2, Lhs.3, _
                      {6}    | REWRITE WITH Tmp.5 := 1, Out.5 := (InOut.1 - Tmp.5), TEST Out.5 = InOut.3
           0   ~0%    {4}    | SCAN OUTPUT In.2, In.0, In.1, In.4

          91   ~3%    {6} r3 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Lhs.1, Lhs.2, Lhs.3, _
                      {6}    | REWRITE WITH Tmp.5 := 1, Out.5 := (InOut.1 - Tmp.5), TEST Out.5 = InOut.3
          81   ~0%    {4}    | SCAN OUTPUT In.4, In.0, In.1, In.2
          81   ~3%    {4}    | JOIN WITH containerparent_10#join_rhs ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.2, Rhs.1

         269   ~0%    {7} r4 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs#2` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Rhs.2, Lhs.1, Lhs.2, Lhs.3, _
                      {7}    | REWRITE WITH Tmp.6 := 1, Out.6 := (InOut.1 - Tmp.6), TEST Out.6 = InOut.4
          39   ~1%    {5}    | SCAN OUTPUT In.5, In.0, In.1, In.2, In.3
        1295   ~0%    {6}    | JOIN WITH containerparent ON FIRST 1 OUTPUT Rhs.1, Lhs.3, _, Lhs.1, Lhs.2, Lhs.4
        1295   ~0%    {6}    | REWRITE WITH Out.2 := 1
          34   ~1%    {4}    | JOIN WITH `cached_FileSystem::Container.splitAbsolutePath/2#dispred#dc97b0cc` ON FIRST 3 OUTPUT Lhs.5, Lhs.3, Lhs.4, Lhs.0

         115   ~0%    {4} r5 = r2 UNION r3 UNION r4
         115   ~0%    {4}    | AND NOT `FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d#prev`(FIRST 4)
                      return r5
```

After
```
Evaluated relational algebra for predicate FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d@4fb6e6v7 on iteration 2 running pipeline standard with tuple counts:
        120   ~0%    {4} r1 = SCAN `FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d#prev_delta` OUTPUT In.1, In.0, In.2, In.3

          0   ~0%    {6} r2 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs#1` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Lhs.1, Lhs.2, Lhs.3, _
                     {6}    | REWRITE WITH Tmp.5 := 1, Out.5 := (InOut.1 - Tmp.5), TEST Out.5 = InOut.3
          0   ~0%    {4}    | SCAN OUTPUT In.2, In.0, In.1, In.4

         91   ~0%    {6} r3 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Lhs.1, Lhs.2, Lhs.3, _
                     {6}    | REWRITE WITH Tmp.5 := 1, Out.5 := (InOut.1 - Tmp.5), TEST Out.5 = InOut.3
         81   ~0%    {4}    | SCAN OUTPUT In.4, In.0, In.1, In.2
         81   ~5%    {4}    | JOIN WITH containerparent_10#join_rhs ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.2, Rhs.1

        269   ~0%    {7} r4 = JOIN r1 WITH `_FileSystem::Folder::Append<PathResolution::shouldAppend>::getComponent/2#a6e36a04#join_rhs#2` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Rhs.2, Lhs.1, Lhs.2, Lhs.3, _
                     {7}    | REWRITE WITH Tmp.6 := 1, Out.6 := (InOut.1 - Tmp.6), TEST Out.6 = InOut.4
         39   ~3%    {5}    | SCAN OUTPUT In.5, In.2, In.0, In.1, In.3
         34   ~0%    {4}    | JOIN WITH `FileSystem::Folder::Append<PathResolution::shouldAppend>::getAChildContainer/2#2e91feca` ON FIRST 2 OUTPUT Lhs.4, Lhs.2, Lhs.3, Rhs.2

        115   ~2%    {4} r5 = r2 UNION r3 UNION r4
        115   ~2%    {4}    | AND NOT `FileSystem::Folder::Append<PathResolution::shouldAppend>::appendStep/3#bed54f6d#prev`(FIRST 4)
                     return r5
```

shared/util/codeql/util/FileSystem.qll

@@ -239,6 +239,12 @@ module Make<InputSig Input> {
         result = 0
       }
 
+      pragma[nomagic]
+      private Container getAChildContainer(Container c, string baseName) {
+        result = c.getAChildContainer() and
+        baseName = result.getBaseName()
+      }
+
       pragma[nomagic]
       private Container appendStep(Folder f, string relativePath, int i) {
         i = -1 and
@@ -253,10 +259,7 @@ module Make<InputSig Input> {
           else
             if comp = "."
             then result = mid
-            else (
-              result = mid.getAChildContainer() and
-              result.getBaseName() = comp
-            )
+            else result = getAChildContainer(mid, comp)
         )
       }