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/query2/SkyQueryEnvironment.java b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
index 0957b81..9bb8fc6 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
@@ -27,6 +27,7 @@
 import com.google.common.collect.Maps;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Sets;
+import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.cmdline.TargetParsingException;
 import com.google.devtools.build.lib.cmdline.TargetPattern;
 import com.google.devtools.build.lib.collect.CompactHashSet;
@@ -36,7 +37,6 @@
 import com.google.devtools.build.lib.packages.NoSuchTargetException;
 import com.google.devtools.build.lib.packages.NoSuchThingException;
 import com.google.devtools.build.lib.packages.Package;
-import com.google.devtools.build.lib.packages.PackageIdentifier;
 import com.google.devtools.build.lib.packages.Rule;
 import com.google.devtools.build.lib.packages.Target;
 import com.google.devtools.build.lib.pkgcache.PathPackageLocator;