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/RecursivePackageProviderBackedTargetPatternResolver.java b/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePackageProviderBackedTargetPatternResolver.java
index 5764b6c..b24ee7a 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePackageProviderBackedTargetPatternResolver.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RecursivePackageProviderBackedTargetPatternResolver.java
@@ -21,7 +21,7 @@
 import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
-import com.google.devtools.build.lib.cmdline.PackageIdentifier.RepositoryName;
+import com.google.devtools.build.lib.cmdline.RepositoryName;
 import com.google.devtools.build.lib.cmdline.ResolvedTargets;
 import com.google.devtools.build.lib.cmdline.TargetParsingException;
 import com.google.devtools.build.lib.cmdline.TargetPatternResolver;