[rubygems/rubygems] Detect extension files under full_required_paths

When we use this methods with local gemspec, we don't handle
  build status of extension correctly. So We need to find extension
  files in require_paths.

  Example with ruby/erb repository:

  ```
  $ bundle exec irb
  Ignoring erb-4.0.2 because its extensions are not built. Try: gem pristine erb --version 4.0.2
  >>
  ```

https://github.com/rubygems/rubygems/commit/f90e43cf3f

lib/rubygems/specification.rb

@@ -12,6 +12,8 @@ require_relative "stub_specification"
 require_relative "platform"
 require_relative "util/list"
 
+require "rbconfig"
+
 ##
 # The Specification class contains the information for a gem.  Typically
 # defined in a .gemspec file or a Rakefile, and looks like this:
@@ -2182,6 +2184,16 @@ class Gem::Specification < Gem::BasicSpecification
     return false if default_gem?
     return false if File.exist? gem_build_complete_path
 
+    # When we use this methods with local gemspec, we don't handle
+    # build status of extension correctly. So We need to find extension
+    # files in require_paths.
+    # TODO: Gem::Specification couldn't access extension name from extconf.rb
+    #       so we find them with heuristic way. We should improve it.
+    return false if (full_require_paths - [extension_dir]).any? do |path|
+      File.exist?(File.join(path, "#{name}.#{RbConfig::CONFIG['DLEXT']}")) ||
+        !Dir.glob(File.join(path, name, "*.#{RbConfig::CONFIG['DLEXT']}")).empty?
+    end
+
     true
   end