Enabled did_you_mean, install during build, and clear gem paths

[headius]

This enables did_you_mean and installs it during the build. Fixes #3480.

@mkristian I could not figure out how to make this work without being a default gem. We do not have any logic to preinstall gems that aren't default, do we?

This also includes the Gem.clear_paths hack to address too-early booting of RubyGems in embedded scenarios.

[mkristian]

@headius what is the advantage of preinstalled gems vs. default gem. a preinstalled gem is gone the moment you set GEM_HOME and GEM_PATH, i.e. you want to install gems with jruby-complete.jar.

[headius]

@mkristian Not a great deal, I guess. Being preinstalled means you won't be able to require it if gems are disabled...that's the only real difference. But for a library being activated immediately, being preinstalled also boots much more of RG than we want.

MRI chose not to make this a default gem, and they are actively trying to move away from default gems. I don't know their reasons, though.

[headius]

I'm open for a vote on what to do from here... merge this now to master as a default gem (and probably remove the gem and Gem.clear_paths calls), or wait and do something different?

@enebo @mkristian @kares

[mkristian]

+1 merge,

we have a number of default gems and the question preinstalled gem vs. default gem is a more general topic

[headius]

Might as well let it bake now.

[yuki24]

I'm very honored to see the did_you_mean gem become part of JRuby. Thank you for making this happen! Please let me know if there's anything I can help. I'm always happy to make JRuby even better!

[headius]

@yuki24 Thank you for working through issues with us!