Move RepositoryName to its own top-level class

As it's about to get more important in its own right.

The only change that isn't just moving code around is making
RepositoryName.validate package-private.

--
MOS_MIGRATED_REVID=112956571
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePkgValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePkgValue.java
index 019de51..8919578 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePkgValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePkgValue.java
@@ -14,7 +14,7 @@
 package com.google.devtools.build.lib.skyframe;
 
 import com.google.common.collect.ImmutableSet;
-import com.google.devtools.build.lib.cmdline.PackageIdentifier.RepositoryName;
+import com.google.devtools.build.lib.cmdline.RepositoryName;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;