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/OutputFile.java b/src/main/java/com/google/devtools/build/lib/packages/OutputFile.java
index b7259da..eed4c63 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/OutputFile.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/OutputFile.java
@@ -38,6 +38,11 @@
     return generatingRule.getVisibility();
   }
 
+  @Override
+  public boolean isConfigurable() {
+    return true;
+  }
+
   /**
    * Returns the rule which generates this output file.
    */