Take into account repository mapping when processing labels inside BUILD files within external repositories.

For example:
a/BUILD
genrule(
  name = "a",
  srcs = ["@x//:x.txt"],
  outs = ["result.txt"],
  cmd = "echo hello > \$(location result.txt)"
)

If the main workspace file references that repository with a rule:
local_repository(
  name = "other_repo",
  path = "../a",
  repo_mapping = {"@x" : "@y"}
)

Then when a/BUILD is evaluated, the string "@x//:x.txt" will be turned into a Label "@y//:x.txt"

RELNOTES: None
PiperOrigin-RevId: 201562148
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 d6f7944..3d84851 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
@@ -926,6 +926,11 @@
       return this;
     }
 
+    /** Get the repository mapping for this package */
+    ImmutableMap<RepositoryName, RepositoryName> getRepositoryMapping() {
+      return this.repositoryMapping;
+    }
+
     /**
      * Sets the name of this package's BUILD file.
      */