Split FileDependencySerializer.NodeDependencyHandler.getValue into phases.

Pure refactoring; no behavior change.

getValue had grown to about 210 lines across the transitive reduction and
bitmap reference counting changes. It is now a short outline:

- collectNodeDependencies gathers the nested and remote children.
- reduce performs ordering and transitive reduction over child nodes and
  direct leaves, returning a ReducedDependencies. It is the only
  method that mutates the leaf closure, so the closure is final once it
  returns. Previously, a comment guarded where it became safe to canonicalize.
- unwrap handles the constant and unary cases.
- getValue releases child bitmaps at a single point, rather than at each of
  three exits, so a future early return cannot strand child references.
- publish serializes and writes the node.

PiperOrigin-RevId: 1005909955
Change-Id: I127adf7463d8cd3183ceee6e0d9caaf202d59996