Consider to make the Gem.default_path more static

[voxik]

The current implementation of Gem.default_path is quite dynamic:

rubygems/lib/rubygems/defaults.rb

Lines 175 to 181 in 0339d53

	def self.default_path
	path = []
	path << user_dir if user_home && File.exist?(user_home)
	path << default_dir
	path << vendor_dir if vendor_dir && File.directory?(vendor_dir)
	path
	end

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)