Move PackageIdentifier to cmdline

This is necessary to have TargetResolver depend on it without making it depend
on the packages target. First step of #389.

--
MOS_MIGRATED_REVID=101790345
diff --git a/src/main/java/com/google/devtools/build/lib/packages/GlobCache.java b/src/main/java/com/google/devtools/build/lib/packages/GlobCache.java
index af596ff..d459bf6 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/GlobCache.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/GlobCache.java
@@ -21,6 +21,7 @@
 import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
 import com.google.common.util.concurrent.SettableFuture;
+import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.concurrent.ThreadSafety;
 import com.google.devtools.build.lib.util.Pair;
 import com.google.devtools.build.lib.vfs.Path;