Ensure version from bundled_gems is used in tool/rdoc-srcdir (ruby#16712)

Use version from bundled_gems in tool/rdoc-srcdir

Previously, `tool/rdoc-srcdir` used `Dir.glob(...).first` to find
bundled gems like rdoc and tsort. This picks the first match
alphabetically, which can select a stale older version when multiple
versions coexist in `.bundle/gems/` (e.g. `rdoc-7.1.0` over
`rdoc-7.2.0`).

Fix by reading the declared version from `gems/bundled_gems` and
constructing the exact path, ensuring the correct version is always
loaded regardless of leftover directories.

Author: st0012

tool/rdoc-srcdir

@@ -1,7 +1,16 @@
 #!ruby -W0
 
+srcdir = File.dirname(__dir__)
+bundled_gems = File.join(srcdir, "gems/bundled_gems")
+versions = {}
+File.foreach(bundled_gems) do |line|
+  next if line.start_with?("#") || line.strip.empty?
+  name, version, = line.split
+  versions[name] = version
+end
+
 %w[tsort rdoc].each do |lib|
-  path = Dir.glob("#{File.dirname(__dir__)}/.bundle/gems/#{lib}-*").first
+  path = File.join(srcdir, ".bundle/gems/#{lib}-#{versions[lib]}")
   $LOAD_PATH.unshift("#{path}/lib")
 end
 require 'rdoc/rdoc'