Factor out TargetRegistrationEnvironment from Package.Builder

`Package.java` is too large and its builder class tries to do too many things. This and the following CLs are an effort to reduce its LOC and better separate concerns.
The main strategy for accomplishing this is to move out the logic for registering targets/macros, validating their names, and indexing them, into a separate dedicated container class.

Future work could look at minimizing `Package.Builder` down to just the things needed to construct a `Package` in the most straightforward context, deserialization. All the stuff that's needed to evaluate BUILD Starlark threads doesn't have to be there -- for instance, `glob()` and `generator_name` machinery.

This CL is focused on just branching `Package.Builder` into a new superclass, `TargetRegistrationEnvironment`. To keep the diff simple I avoided other refactoring in this CL, such as reordering members and restricting field access by adding new getters. The net result is about 600 LOC moved out of a 2800 line file.

In Package.java:

- Inlined static methods `getTargets(BiMap) and `getTargets(Target, Class)`.

- Moved the map fields that track registration of targets and macros. Also moved `currentMacroFrame`, `nameConflictCheckingPolicy`, the `ruleLabels` cache, and `containsErrors`. I did *not* move `unexpandedMacros` and `rulesSnapshotViewForFinalizers` because those are part of the evaluation model for symbolic macros, and aren't needed for their registration / conflict checking.

- Moved corresponding accessors/setters for these fields.

- The new base class doesn't track `pkg`, so it can't do the precondition checks that the targets it's manipulating belong to the package being constructed. For now, I kept these checks by overriding the applicable methods in the Builder class and dispatching to `super`. If these checks are worthwhile, then the better solution is to add a `pkg` field to `TargetRegistrationEnvironment`.

- Methods that *create* targets (rather than merely adding them) are *not* pushed up to `TargetRegistrationEnvironment`.

In TargetRegistrationEnvironment:

- Fields access broadened (for now) from `private` to `protected`. (Technically, package-private would've also worked, but `protected` is a clear signal that it's intended for use in a subclass.) Same for a few methods.

- Update javadoc/comment in `setContainsErrors()` to not mention `addEvent[s]()`, which hasn't existed on `Package.Builder` since d8d9078.

- `disableNameConflictChecking()` now returns void.

Work toward #19922.

PiperOrigin-RevId: 678399563
Change-Id: I32cfd2c1d1aab142f271235abd59fd9d46cabf9d

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

@@ -23,8 +23,8 @@
 import com.google.common.collect.Lists;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.events.Event;
-import com.google.devtools.build.lib.packages.Package.Builder.MacroFrame;
 import com.google.devtools.build.lib.packages.TargetDefinitionContext.NameConflictException;
+import com.google.devtools.build.lib.packages.TargetRegistrationEnvironment.MacroFrame;
 import com.google.devtools.build.lib.server.FailureDetails.PackageLoading.Code;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.util.ArrayList;