Intern empty Depsets

[fmeum]

Empty Depsets are interned per order, which decreases allocations as well as retained heap when a Depset is stored in a struct in a provider (providers with schema already unwrap most Depsets).

Fixes #19380

[comius]

Is it possible to follow the same pattern as for NestedSets? That is to add emptyDepset next to emptySet to Order class.

That would satisfy the requirement to have an empty depset interned for each Order element.

It also seems not to create a problem with a cycle, because Depset, Nestedset and Order are in the same java_library.

Implementing it in Order, would make the code a bit simpler (not introducing 2 different implementation for NestedSet and Depset) and making it easier to follow.

[fmeum]

Done. This required some further changes to break cyclic dependencies between Depset's and Order's static initializers.

Let me know if I should also add @SerializationConstants for the empty depsets - I don't know whether they could be subject to serialization.

[fmeum]

@bazel-io flag

[iancha1992]

@bazel-io fork 6.4.0

[iancha1992]

@fmeum @comius
We tried to cherry-pick to release-6.4.0. But there are some merge conflicts with src/main/java/com/google/devtools/build/lib/collect/nestedset/Depset.java

1. We currently have in release-6.4.0 below

private Depset(ElementType elemType, NestedSet<?> set) {
    this.elemType = Preconditions.checkNotNull(elemType, "element type cannot be null");
    this.set = set;
  }

But, we expected before the cherry-pick to be:

private Depset(@Nullable Class<?> elemClass, NestedSet<?> set) {
    this.elemClass = elemClass;
    this.set = set;
}

2. We expected before the cherry-pick:

public static <T> Depset of(Class<T> elemClass, NestedSet<T> set) {
    Preconditions.checkNotNull(elemClass, "elemClass cannot be null");
    // Having a shared EMPTY_DEPSET instance, could reduce working heap. Empty depsets are not
    // retained though, because of optimization in StarlarkProvider.optimizeField.
    return new Depset(set.isEmpty() ? null : ElementType.getTypeClass(elemClass), set);
}

but we currently have in the release-6.4.0 branch, below:

public static <T> Depset of(ElementType elemType, NestedSet<T> set) {
    return new Depset(elemType, set);
}

cc: @bazelbuild/triage

[fmeum]

@iancha1992 I submitted a resolution of these conflicts as #19443.