Make definition nesting usable for constant resolution

[soutaro]

Summary

Fix Graph#resolve_constant for lexical nesting stacks containing rooted or fully qualified namespace names.

Background

When callers construct a nesting stack from Definition#lexical_nesting, explicit qualification affects constant lookup. For example:

module Foo
  module ::Bar
    CONST
  end
end

The inner frame must remain ::Bar; treating it as Bar would incorrectly resolve it relative to Foo.

Graph#resolve_constant creates a temporary Name chain from the supplied strings. Rooted and fully qualified frames can introduce parent-scope and lexical-nesting dependencies that have not yet been resolved. Previously, only the final name was resolved, so lookup could fail even when the corresponding declarations existed in the graph.

Changes

• Add recursive constant resolution that resolves a name's lexical nesting and parent scope before resolving the name itself.
• Use recursive resolution from the Ruby Graph#resolve_constant API.
• Add source_name to class, module, constant, and constant-alias definitions. Unlike name, it preserves explicit source-level qualification such as Bar, Foo::Bar, or ::Bar, allowing callers to construct constant-resolution nesting stacks without inferring qualification from resolved declarations.