Move stdlib/rubygems/ to core/rubygems/

[pocke]

I propose to move RubyGems library's RBS from stdlib to core.

In Ruby, rubygems is loaded by default unless --disable-gems is given. --disable-gems option is provided only for debugging Ruby core. So we can suppose rubygems is loaded always.

But currently RBS provides rubygems as a standard library, so the user needs to load rubygems explicitly in RBS. It is different from Ruby.

So I think RBS should provide rubygems as a core library.

Cons

• Performance
  • Loading RBSs will be slower by this change to parse/load rubygems.rbs.
  • I guess the parsing cost is small so we can ignore it.
• Compatibility
  • If rubygems library is used in existing code, it will causes a load error (or something) by this change.
  • For example, library 'rubygems' in Steepfile will raise an error if rubygems library is gone.

[soutaro]

Sounds good! 👍