This decouples `gem exec` behavior (and tests) from the sort order of
the gemspec.

https://github.com/rubygems/rubygems/commit/911cd29159

When `gem exec foo` is run, and "foo" is a gem that has multiple
executables, none of them named "foo", raise an error explaining the
situation and telling user to be more specific.

Currently the first command in the executables array is run, but this
may come as surprising sometimes, so better raise an error.

https://github.com/rubygems/rubygems/commit/acda5d8f6e

The $0 value is used in many CLI libraries to determine the name of the
application, when displaying help and error messages.

Without setting this value, it defaults to `gem` which can be confusing.

Before:

```
$ gem exec kamal help
Commands:
  gem accessory           # Manage accessories (db/redis/search)
  gem app                 # Manage application
  gem audit               # Show audit log from servers
  gem build               # Build application image
  gem config              # Show combined config (including secrets!)
  gem deploy              # Deploy app to servers
  gem details             # Show details about all containers
  gem docs [SECTION]      # Show Kamal configuration documentation
  gem help [COMMAND]      # Describe available commands or one specific command
  gem init                # Create config stub in config/deploy.yml and secrets stub in .kamal
  gem lock                # Manage the deploy lock
  gem proxy               # Manage kamal-proxy
  gem prune               # Prune old application images and containers
  gem redeploy            # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
  gem registry            # Login and -out of the image registry
  gem remove              # Remove kamal-proxy, app, accessories, and registry session from servers
  gem rollback [VERSION]  # Rollback app to VERSION
  gem secrets             # Helpers for extracting secrets
  gem server              # Bootstrap servers with curl and Docker
  gem setup               # Setup all accessories, push the env, and deploy app to servers
  gem upgrade             # Upgrade from Kamal 1.x to 2.0
  gem version             # Show Kamal version
```

After:

```
$ gem exec kamal help
Commands:
  kamal accessory           # Manage accessories (db/redis/search)
  kamal app                 # Manage application
  kamal audit               # Show audit log from servers
  kamal build               # Build application image
  kamal config              # Show combined config (including secrets!)
  kamal deploy              # Deploy app to servers
  kamal details             # Show details about all containers
  kamal docs [SECTION]      # Show Kamal configuration documentation
  kamal help [COMMAND]      # Describe available commands or one specific command
  kamal init                # Create config stub in config/deploy.yml and secrets stub in .kamal
  kamal lock                # Manage the deploy lock
  kamal proxy               # Manage kamal-proxy
  kamal prune               # Prune old application images and containers
  kamal redeploy            # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
  kamal registry            # Login and -out of the image registry
  kamal remove              # Remove kamal-proxy, app, accessories, and registry session from servers
  kamal rollback [VERSION]  # Rollback app to VERSION
  kamal secrets             # Helpers for extracting secrets
  kamal server              # Bootstrap servers with curl and Docker
  kamal setup               # Setup all accessories, push the env, and deploy app to servers
  kamal upgrade             # Upgrade from Kamal 1.x to 2.0
  kamal version             # Show Kamal version
```

https://github.com/rubygems/rubygems/commit/4fd060b96d

https://github.com/rubygems/rubygems/commit/511c7b211b

https://github.com/rubygems/rubygems/commit/032b3c518a

https://github.com/rubygems/rubygems/commit/d8efd919db

https://github.com/rubygems/rubygems/commit/f177990d4c

Gem.path_separator can also be a regexp...

https://github.com/rubygems/rubygems/commit/7e6e56a5f3

Will save on an error if it would be nil from an exception happening during that line

https://github.com/rubygems/rubygems/commit/d6797a04e9