Always rely on $LOAD_PATH when jumping from exe/ to lib/

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

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)

What is your fix for the problem, implemented in this PR?

Changing exe/bundle to always use $LOAD_PATH.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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