Don't zero-out builder fields in finishBuild()

The builder itself shouldn't be retained after package construction, so there shouldn't be a need to nullify its fields. Running the benchmark script shows this CL has no effect on memory.

The line wrapping `targets` in an unmodifiable map dates back to unknown commit, and at that point was needed because Package didn't copy the map itself in `finishInit()` but just pointed to the builder's map.

Eliminating these nullifications makes it easier to make these fields `private` in `TargetRegistrationEnvironment`.

Work toward #19922.

PiperOrigin-RevId: 678401129
Change-Id: Ica887b69fb795aaebf00d86e666b2817aa0ef659

src/main/java/com/google/devtools/build/lib/packages/Package.java

@@ -2092,14 +2092,10 @@ public Package finishBuild() {
         return pkg;
       }
 
-      // Freeze targets and distributions.
+      // Freeze rules, compacting their attributes' representations.
       for (Rule rule : getRules()) {
         rule.freeze();
       }
-      ruleLabels = null;
-      rulesCreatedInMacros = null;
-      outputFilePrefixes = null;
-      targets = Maps.unmodifiableBiMap(targets);
 
       // Now all targets have been loaded, so we validate the group's member environments.
       for (EnvironmentGroup envGroup : ImmutableSet.copyOf(environmentGroups.values())) {

src/main/java/com/google/devtools/build/lib/packages/TargetRegistrationEnvironment.java

@@ -167,7 +167,7 @@ private enum NameConflictCheckingPolicy {
    * <p>This field is null if name conflict checking is disabled. It is also null after the package
    * is built. The content of the map is manipulated only in {@link #checkRuleAndOutputs}.
    */
-  @Nullable protected Map<String, OutputFile> outputFilePrefixes = new HashMap<>();
+  @Nullable private Map<String, OutputFile> outputFilePrefixes = new HashMap<>();
 
   protected TargetRegistrationEnvironment(RepositoryMapping mainRepositoryMapping) {
     super(mainRepositoryMapping);