Factor out constants in Label class.

This is to be able to create an additional target in the cmdline package to split a cycle in unknown commit.

RELNOTES: None
PiperOrigin-RevId: 225409893
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java b/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
index 732e6a0..4db5bf6 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
@@ -86,7 +86,7 @@
         ? Preconditions.checkNotNull(
             execPath.getParentDirectory(), "Must pass in files, not root directory")
         : execPath;
-    if (tofind.startsWith(Label.EXTERNAL_PATH_PREFIX)) {
+    if (tofind.startsWith(LabelConstants.EXTERNAL_PATH_PREFIX)) {
       // TODO(ulfjack): Remove this when kchodorow@'s exec root rearrangement has been rolled out.
       RepositoryName repository = RepositoryName.create("@" + tofind.getSegment(1));
       return PackageIdentifier.create(repository, tofind.subFragment(2));