Parse the label syntax "@//a:b" so that eventually we can make that the syntax that means "refer to the main repository".

There isn't an overarching plan for what we are going to do with the cmdline package, which seems to be separated from the .syntax one in all sorts of awkward ways.

--
MOS_MIGRATED_REVID=103088960
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleDefinitionEnvironment.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleDefinitionEnvironment.java
index 4dcd080..1f00eca 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleDefinitionEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleDefinitionEnvironment.java
@@ -23,9 +23,7 @@
 public interface RuleDefinitionEnvironment {
   /**
    * Parses the given string as a label and returns the label, by calling {@link
-   * Label#parseAbsolute}. Instead of throwing a {@link
-   * com.google.devtools.build.lib.syntax.Label.SyntaxException}, it throws an {@link
-   * IllegalArgumentException}, if the parsing fails.
+   * Label#parseAbsolute}. Throws a {@link IllegalArgumentException} if the parsing fails.
    */
   Label getLabel(String labelValue);
 }