Update comment about repository mapping containing entry from "@<main repo name>" to "@".

RELNOTES: None
PiperOrigin-RevId: 207010178

src/main/java/com/google/devtools/build/lib/packages/Package.java

diff --git a/src/main/java/com/google/devtools/build/lib/packages/Package.java b/src/main/java/com/google/devtools/build/lib/packages/Package.java
index 82b2aca..daefc112 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Package.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Package.java
@@ -815,7 +815,8 @@
     private final HashMap<RepositoryName, HashMap<RepositoryName, RepositoryName>>
         externalPackageRepositoryMappings = new HashMap<>();
     // The map of repository reassignments for BUILD packages loaded within external repositories.
-    // It will be empty for packages within the main workspace.
+    // It contains an entry from "@<main workspace name>" to "@" for packages within
+    // the main workspace.
     private ImmutableMap<RepositoryName, RepositoryName> repositoryMapping = ImmutableMap.of();
     private Path filename = null;
     private Label buildFileLabel = null;