`bundle exec pod install` fails with: LoadError - cannot load such file -- rexml/document

Nobody has claimed this yet.

### Command

```
/usr/local/lib/ruby/gems/3.0.0/bin/pod install
```

### Report

* What did you do?

I tried building from the latest sources. At first it complained about `bundle install` so I chose to update it:

```
$ bundle install
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/rubygems.rb:281:in `find_spec_for_exe': Could not find 'bundler' (1.17.3) required by your /Users/rmann/Projects/Personal/MacDown/upstream/Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update --bundler`.
To install the missing version, run `gem install bundler:1.17.3`
from /usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/rubygems.rb:300:in `activate_bin_path'
from /usr/local/opt/ruby/bin/bundle:23:in `'
8572 22:24: upstream/ (master) $ bundle update --bundler
```

Perhaps it didn't like that.

* What did you expect to happen?

* What happened instead?

### Stack

```
CocoaPods : 1.10.1
Ruby : ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-darwin19]
RubyGems : 3.2.15
Host : macOS 11.3 (20E232)
Xcode : 12.5 (12E262)
Git : git version 2.28.0
Ruby lib dir : /usr/local/Cellar/ruby/3.0.1/lib
Repositories : macdownapp - git - https://github.com/MacDownApp/cocoapods-specs.git @ 8db889cd3472ec88521227a7eff5d50fd066fcc8

master - git - https://github.com/CocoaPods/Specs.git @ 5834d682b8bca7efd81969a17ea68e3ad4ca3073

trunk - CDN - https://cdn.cocoapods.org/
```

### Plugins

```
cocoapods-deintegrate : 1.0.4
cocoapods-plugins : 1.0.0
cocoapods-search : 1.0.0
cocoapods-trunk : 1.5.0
cocoapods-try : 1.2.0
```

### Podfile

```ruby
platform :osx, "10.8"

source 'https://github.com/MacDownApp/cocoapods-specs.git' # Patched libraries.
source 'https://cdn.cocoapods.org/'

project 'MacDown.xcodeproj'

inhibit_all_warnings!

target "MacDown" do
pod 'handlebars-objc', '~> 1.4'
pod 'hoedown', '~> 3.0.7', :inhibit_warnings => false
pod 'JJPluralForm', '~> 2.1'
pod 'LibYAML', '~> 0.1'
pod 'M13OrderedDictionary', '~> 1.1'
pod 'MASPreferences', '~> 1.3'
pod 'Sparkle', '~> 1.18', :inhibit_warnings => false

# Locked on 0.4.x until we drop 10.8.
pod 'PAPreferences', '~> 0.4'
end

target "MacDownTests" do
pod 'PAPreferences', '~> 0.4'
end

target "macdown-cmd" do
pod 'GBCli', '~> 1.1'
end
```

### Error

```
LoadError - cannot load such file -- rexml/document
/usr/local/lib/ruby/gems/3.0.0/gems/xcodeproj-1.19.0/lib/xcodeproj/workspace.rb:2:in `require'
/usr/local/lib/ruby/gems/3.0.0/gems/xcodeproj-1.19.0/lib/xcodeproj/workspace.rb:2:in `'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/user_project_integrator.rb:1:in `require'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/user_project_integrator.rb:1:in `'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator/pod_target_integrator.rb:219:in `require'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator/pod_target_integrator.rb:219:in `add_copy_dsyms_script_phase'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator/pod_target_integrator.rb:39:in `block in integrate!'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/user_interface.rb:64:in `section'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator/pod_target_integrator.rb:33:in `integrate!'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator.rb:157:in `block (2 levels) in integrate_targets'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator.rb:156:in `each'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator.rb:156:in `block in integrate_targets'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/user_interface.rb:145:in `message'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/pods_project_generator.rb:154:in `integrate_targets'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/xcode/single_pods_project_generator.rb:23:in `generate!'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:312:in `block in create_and_save_projects'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/user_interface.rb:64:in `section'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:307:in `create_and_save_projects'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:299:in `generate_pods_project'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:178:in `integrate'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:166:in `install!'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/command/install.rb:52:in `run'
/usr/local/lib/ruby/gems/3.0.0/gems/claide-1.0.3/lib/claide/command.rb:334:in `run'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/lib/cocoapods/command.rb:52:in `run'
/usr/local/lib/ruby/gems/3.0.0/gems/cocoapods-1.10.1/bin/pod:55:in `'
/usr/local/lib/ruby/gems/3.0.0/bin/pod:23:in `load'
/usr/local/lib/ruby/gems/3.0.0/bin/pod:23:in `'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli/exec.rb:63:in `load'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli/exec.rb:63:in `kernel_load'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli/exec.rb:28:in `run'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli.rb:494:in `exec'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli.rb:30:in `dispatch'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/cli.rb:24:in `start'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.15/exe/bundle:49:in `block in '
/usr/local/Cellar/ruby/3.0.1/lib/ruby/3.0.0/bundler/friendly_errors.rb:130:in `with_friendly_errors'
/usr/local/Cellar/ruby/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.15/exe/bundle:37:in `'
/usr/local/opt/ruby/bin/bundle:23:in `load'
/usr/local/opt/ruby/bin/bundle:23:in `'
```

Start with the reported bundle install and bundle exec pod install commands, then inspect Gemfile.lock and the Podfile. Trace the LoadError from xcodeproj-1.19.0/lib/xcodeproj/workspace.rb and verify the dependency setup against the reported Ruby and CocoaPods versions. Done means pod install completes without the rexml/document error and the project dependencies are generated.

Written by the indexing model from the issue text.