Make MissingDepException static

No need for it to be otherwise.

--
MOS_MIGRATED_REVID=96437024
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/EnvironmentBackedRecursivePackageProvider.java b/src/main/java/com/google/devtools/build/lib/skyframe/EnvironmentBackedRecursivePackageProvider.java
index 938c8d4..0acc5e0 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/EnvironmentBackedRecursivePackageProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/EnvironmentBackedRecursivePackageProvider.java
@@ -117,6 +117,6 @@
    * method in {@link com.google.devtools.build.lib.skyframe.TargetPatternFunction}, which then
    * returns null in accordance with the skyframe missing dependency policy.
    */
-  class MissingDepException extends RuntimeException {
+  static class MissingDepException extends RuntimeException {
   }
 }