Normally, we use `require_relative` when requiring internal code.
However, Bundler may also load external code (for example, bundler
plugins), and when that external code loads Bundler itself, using
`require_relative` without setting up the `$LOAD_PATH` may lead into
double load errors.

For example, when testing a Gemfile using plugins using our source code
tree, I get the following:

```
➜ plugins-dup ~/Code/rubygems/rubygems/bundler/exe/bundle
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using bundler 2.5.0.dev
Installing extended_bundler-errors 0.3.3
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/version.rb:4: warning: already initialized constant Bundler::VERSION
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/version.rb:4: warning: previous definition of VERSION was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/worker.rb:5: warning: already initialized constant Bundler::Worker::POISON
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/worker.rb:5: warning: previous definition of POISON was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:183: warning: already initialized constant Bundler::FileUtils::VERSION
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:183: warning: previous definition of VERSION was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:2165: warning: already initialized constant Bundler::FileUtils::Entry_::S_IF_DOOR
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:2165: warning: previous definition of S_IF_DOOR was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:2461: warning: already initialized constant Bundler::FileUtils::Entry_::DIRECTORY_TERM
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:2461: warning: previous definition of DIRECTORY_TERM was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:2569: warning: already initialized constant Bundler::FileUtils::OPT_TABLE
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:2569: warning: previous definition of OPT_TABLE was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:2627: warning: already initialized constant Bundler::FileUtils::LOW_METHODS
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:2627: warning: previous definition of LOW_METHODS was here
/Users/deivid/.asdf/installs/ruby/3.2.2/lib/ruby/site_ruby/3.2.0/bundler/vendor/fileutils/lib/fileutils.rb:2634: warning: already initialized constant Bundler::FileUtils::METHODS
/Users/deivid/Code/rubygems/rubygems/bundler/lib/bundler/vendor/fileutils/lib/fileutils.rb:2634: warning: previous definition of METHODS was here
Failed to install plugin `extended_bundler-errors`, due to Bundler::Plugin::MalformattedPlugin (ArgumentError: Trying to register Bundler::GemfileError for status code 4 but
Bundler::GemfileError is already registered)
```

Changing this to always use `$LOAD_PATH` fixes the problem.

https://github.com/rubygems/rubygems/commit/121a861c39