Consider to make the `Gem.default_path` more static

[voxik]

The current implementation of Gem.default_path is quite dynamic:

https://github.com/rubygems/rubygems/blob/0339d53cd5fcece4504848c784d8fd9756ea4502/lib/rubygems/defaults.rb#L175-L181

It does not include user_dir and vendor_dir under some conditions, mainly if the directories are not present on the filesystem. That IMHO makes information about versatility of RubyGems more obscure.

Therefore I propose to make the Gem.default_path more static, starting by removing the directory existence check.

Please note that this ticket is based on discussion in #7187 (comment)