Don't crash when building environment groups directly
(following PackageGroup's precedent).

Also cleanup: generalize the pattern by which we determine
non-configurable target types.

--
MOS_MIGRATED_REVID=104125803
diff --git a/src/main/java/com/google/devtools/build/lib/packages/PackageGroup.java b/src/main/java/com/google/devtools/build/lib/packages/PackageGroup.java
index d7fe3df..3695e36 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/PackageGroup.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/PackageGroup.java
@@ -146,6 +146,11 @@
     return ConstantRuleVisibility.PUBLIC;
   }
 
+  @Override
+  public boolean isConfigurable() {
+    return false;
+  }
+
   public static String targetKind() {
     return "package group";
   }