Simplify BlazeModule.beforeCommand

Don't pass the Command annotation explicitly, but add it to CommandEnvironment
instead; most modules don't need it in the first place, so it was a lot of
boilerplate for not much. Also change it so that the command is passed to the
constructor.

Add some documentation to the beforeCommand method.

PiperOrigin-RevId: 158847128
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java b/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
index 0061e3f..ee2ca95 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
@@ -44,7 +44,7 @@
   private boolean discardActions;
 
   @Override
-  public void beforeCommand(Command command, CommandEnvironment env) {
+  public void beforeCommand(CommandEnvironment env) {
     this.reporter = env.getReporter();
     this.eventBus = env.getEventBus();
     eventBus.register(this);