Fallback to copy symlinks on Windows

[larskanis]

Symlinks are not permitted by default for a Windows user. To use them, a switch called "Development Mode" in the system settings has to be enabled.

What was the end-user or developer problem that led to this PR?

Ordinary users as well as administrators are unable per default to install gems using symlinks.
One such problematical gem is haml-rails-3.0.0.
It uses symlinks for files and directories.
The resulting error message is not very helpful:

$ gem inst haml-rails
Fetching haml-rails-3.0.0.gem
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the directory. (Gem::FilePermissionError)
        C:/ruby/lib/ruby/4.0.0/rubygems/installer.rb:308:in 'Gem::Installer#install'
        C:/ruby/lib/ruby/4.0.0/rubygems/resolver/specification.rb:105:in 'Gem::Resolver::Specification#install'
        C:/ruby/lib/ruby/4.0.0/rubygems/request_set.rb:192:in 'block in Gem::RequestSet#install'
        C:/ruby/lib/ruby/4.0.0/rubygems/request_set.rb:183:in 'Array#each'
        C:/ruby/lib/ruby/4.0.0/rubygems/request_set.rb:183:in 'Gem::RequestSet#install'
        C:/ruby/lib/ruby/4.0.0/rubygems/commands/install_command.rb:207:in 'Gem::Commands::InstallCommand#install_gem'
        C:/ruby/lib/ruby/4.0.0/rubygems/commands/install_command.rb:223:in 'block in Gem::Commands::InstallCommand#install_gems'
        C:/ruby/lib/ruby/4.0.0/rubygems/commands/install_command.rb:216:in 'Array#each'
        C:/ruby/lib/ruby/4.0.0/rubygems/commands/install_command.rb:216:in 'Gem::Commands::InstallCommand#install_gems'
        C:/ruby/lib/ruby/4.0.0/rubygems/commands/install_command.rb:162:in 'Gem::Commands::InstallCommand#execute'
        C:/ruby/lib/ruby/4.0.0/rubygems/command.rb:326:in 'Gem::Command#invoke_with_build_args'
        C:/ruby/lib/ruby/4.0.0/rubygems/command_manager.rb:252:in 'Gem::CommandManager#invoke_command'
        C:/ruby/lib/ruby/4.0.0/rubygems/command_manager.rb:193:in 'Gem::CommandManager#process_args'
        C:/ruby/lib/ruby/4.0.0/rubygems/command_manager.rb:151:in 'Gem::CommandManager#run'
        C:/ruby/lib/ruby/4.0.0/rubygems/gem_runner.rb:56:in 'Gem::GemRunner#run'
        C:/ruby/bin/gem.cmd:20:in '<main>'

What is your fix for the problem, implemented in this PR?

Instead of working around the situation in the affected gem or to skip symlinks completely, I think the better solution would be to make copies of the files in question. This would allow Windows users to install and use the gem smoothly.

I didn't adjust the rubygems tests to support this non-developer-mode use case, because I want to ask if this approach is acceptable, first.

The switch for the "Developer Mode" is available in the Windows registry under
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\AppModelUnlock
entry
AllowDevelopmentWithoutDevLicense

Maybe we could add a Github Action run with AllowDevelopmentWithoutDevLicense=0, and adjust the tests to succeed in both cases.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags