Simplify constants' :: prefix handling

[st0012]

Motivation

As discussed in #893 (comment), we could simplify visitor's :: handling by moving it fully_qualify_name.

Implementation

I decided to handle prefix in fully_qualify_name instead of Index#initialize because Index#initialize wouldn't be able to deal with the name pushed to IndexVisitor's stack.

Index.<< is also not ideal as when it's called entry.name would already have the prefix.

[github-actions]

Benchmark results in seconds (slowest at top)

          textDocument/completion average: 0.267597 std_dev: 0.005312
          textDocument/diagnostic average: 0.043923 std_dev: 0.010385
          textDocument/definition average: 0.005631 std_dev: 0.003163
      textDocument/selectionRange average: 0.004271 std_dev: 0.000616
   textDocument/documentHighlight average: 0.002638 std_dev: 0.000275
      textDocument/documentSymbol average: 0.002574 std_dev: 0.000179
 textDocument/semanticTokens/full average: 0.002565 std_dev: 0.000389
            textDocument/codeLens average: 0.002559 std_dev: 0.000261
        textDocument/documentLink average: 0.002555 std_dev: 0.000198
        textDocument/foldingRange average: 0.002482 std_dev: 0.000202
textDocument/semanticTokens/range average: 0.001689 std_dev: 8.8e-05
               codeAction/resolve average: 0.001481 std_dev: 0.00014
               textDocument/hover average: 0.001425 std_dev: 0.000132
           textDocument/inlayHint average: 0.001422 std_dev: 0.000122
          textDocument/formatting average: 0.000856 std_dev: 0.000318
    textDocument/onTypeFormatting average: 0.000856 std_dev: 0.000103
          textDocument/codeAction average: 0.000836 std_dev: 7.5e-05


================================================================================
Comparison with main branch:

 textDocument/semanticTokens/full unchanged
textDocument/semanticTokens/range unchanged
      textDocument/documentSymbol unchanged
        textDocument/foldingRange unchanged
          textDocument/formatting unchanged
          textDocument/diagnostic unchanged
        textDocument/documentLink unchanged
           textDocument/inlayHint unchanged
      textDocument/selectionRange unchanged
   textDocument/documentHighlight unchanged
               textDocument/hover unchanged
          textDocument/codeAction unchanged
    textDocument/onTypeFormatting unchanged
               codeAction/resolve unchanged
          textDocument/completion unchanged
            textDocument/codeLens unchanged
          textDocument/definition unchanged


================================================================================
Missing benchmarks:

RubyLsp::Requests::ShowSyntaxTree