Lazy-require set and tmpdir to avoid polluting Ruby 2.7 ancestors

Top-level require of 'set' and 'tmpdir' caused additional modules to
be loaded at startup, shifting the Integer ancestor count on Ruby 2.7
and breaking DAP protocol hover tests. Replace Set with plain Hash in
reconcile_breakpoints and lazy-require tmpdir in ensure_wk_lock!.

Author: st0012

lib/debug/session.rb

@@ -42,8 +42,6 @@
 
 require 'json' if ENV['RUBY_DEBUG_TEST_UI'] == 'terminal'
 require 'pp'
-require 'set'
-require 'tmpdir'
 
 class RubyVM::InstructionSequence
   def traceable_lines_norec lines
@@ -113,19 +111,19 @@ def serialize_sync_breakpoints
     end
 
     def reconcile_breakpoints(specs)
-      remote_keys = Set.new
+      remote_keys = {}
 
       specs.each do |spec|
         key = bp_key_from_spec(spec)
         next unless key
-        remote_keys << key
+        remote_keys[key] = true
         unless @bps.key?(key)
           create_bp_from_spec(spec)
         end
       end
 
       @bps.delete_if do |key, bp|
-        if syncable_bp?(bp) && !remote_keys.include?(key)
+        if syncable_bp?(bp) && !remote_keys.key?(key)
           bp.delete
           true
         end
@@ -2163,6 +2161,7 @@ def unlock_wk_lock
 
     private def ensure_wk_lock!
       return if @wk_lock_file
+      require 'tmpdir'
       path = File.join(Dir.tmpdir, "ruby-debug-#{Process.uid}-pgrp-#{Process.getpgrp}.lock")
       @wk_lock_file = File.open(path, File::WRONLY | File::CREAT, 0600)
     rescue SystemCallError => e