Find references from constant definitions

[SlavaEremenko]

Motivation

Find references returns nothing when invoked on a constant's definition, for example with the cursor on STATUSES here:

class Invoice
  STATUSES = %w[open paid].freeze
end

Invoking it on any usage (STATUSES, Invoice::STATUSES) works and even lists the definition, so only the declaration itself is a dead end. Class and module names are unaffected because class Foo holds a ConstantReadNode.

Implementation

The request only located ConstantReadNode, ConstantPathNode and ConstantPathTargetNode. On STATUSES = ..., a ConstantWriteNode, locate fell back to the program node and the request returned early.

ReferenceFinder already collects ConstantWriteNode, ConstantOrWriteNode, ConstantAndWriteNode, ConstantOperatorWriteNode and multi-write ConstantTargetNodes, so the request now accepts the same nodes as starting points and resolves node.name through the current nesting. The resolve-to-ConstTarget step is extracted so both constant branches share it.

While testing the ||= case I found ReferenceFinder registered on_constant_or_write_node_enter with the dispatcher twice, which returned every CONST ||= value as two identical locations no matter where the request started. That's fixed in its own commit.

Two things I noticed but left out to keep this focused:

• Foo::BAR = 1 is also reported twice, because both on_constant_path_write_node_enter and on_constant_path_node_enter fire for the write's target. Those handlers came with rename support, so I didn't want to change them without knowing the intent.
• Rename locates the same three node types, so renaming from a constant's definition has the same gap.

Happy to follow up on either.

Automated Tests

Added request tests for plain, ||=, += and multi-write definitions, plus a ReferenceFinder regression test for the duplicate or-write. bundle exec rake, bundle exec srb tc and bin/rubocop pass locally.

Manual Tests

1. In a workspace, add class Foo; BAR = 1; end to one file and Foo::BAR to another.
2. Right-click BAR on its definition line and pick Find All References.
3. Before: "No references found". After: both the definition and Foo::BAR are listed.