Fix an incorrect constant usage in plantSymlinkForExternalRepo.

LabelConstants.EXTERNAL_PACKAGE_NAME is a conceptual path fragment to represent
the //external package and has nothing to do with the parent directories for
external repo symlinks in the execroot. The correct one is
LabelConstants.EXTERNAL_PATH_PREFIX.

PiperOrigin-RevId: 318048497
diff --git a/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java b/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
index fbaa9ac..c869c1f 100644
--- a/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
+++ b/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
@@ -139,7 +139,7 @@
     Path execrootLink = execroot.getRelative(repository.getExecPath(siblingRepositoryLayout));
 
     if (!siblingRepositoryLayout && externalRepoLinks.isEmpty()) {
-      execroot.getRelative(LabelConstants.EXTERNAL_PACKAGE_NAME).createDirectoryAndParents();
+      execroot.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).createDirectoryAndParents();
     }
     if (!externalRepoLinks.add(execrootLink)) {
       return;