Migrate core package 'encoding-selector' into Atom repo #17841

This issue tracks the steps necessary to migrate the encoding-selector package into the core atom/atom repository under the packages path:

Migration Checklist

Phase 1 - Prepare repo for migration

• Merge or close existing pull requests on the atom/encoding-selector repository

Some guidelines for merging vs closing PRs:

• Merge when the PR is small, useful, has no merge conflicts with master, and has passing tests
• Close when the changes are non-trivial, the PR has conflicts with master, or there are failing tests in CI

If you close a PR, let the user know that we are currently moving the repository over to atom/atom so they should feel free to reopen the pull request there once the package code has been migrated.

Phase 2 - Migrate the package code

• Create a fresh branch off master in your local clone of atom/atom with the name migrate-encoding-selector-package
• Delete Atom's node_modules folder if you've built there before and make sure Atom's package-lock.json file is unmodified
• Clone the atom/encoding-selector repo into the packages path of your local Atom repository and then delete its .git subfolder
• Delete any unnecessary repo configuration files in the packages/encoding-selector folder like appveyor.yml, .travis.yml, ISSUE_TEMPLATE.md, and PULL_REQUEST_TEMPLATE.md
• Remove any information from the package's README.md which will be inaccurate after the migration is complete (e.g. CI status badges or contribution instructions)
• Edit Atom's package.json file to change the following:
  • Update the encoding-selector entry in packageDependencies to reflect the local path of the package:

    "encoding-selector": "file:./packages/encoding-selector",
    

  • Delete the encoding-selector entry in the dependencies section since it may contain a hardcoded tarball link. This will help avoid issues in the next step
  • Change the repository field to point to the main repo URL https://github.com/atom/atom
• Run a full Atom build using script/build and verify that there are no unexpected build errors
• Launch the Atom binary in the ./out folder and manually verify that the migrated package is working without any obvious issues. For example, if you're migrating a color theme, turn on that theme to make sure that it changes the editor colors as expected.
• Open the Chrome Developer Tools console (Ctrl+Shift+I or Cmd+Option+I) and ensure that there are no errors written there. If there are, add them to this issue and investigate.
• Update the second column of the encoding-selector row in packages/README.md to reflect the new local path for the package:

  [`./packages/encoding-selector`](./encoding-selector)
  

• Commit the migrated package code and any changes to Atom's package-lock.json files to your branch using this commit message:

  :arrow_right: Migrate core package 'encoding-selector' into ./packages
  

Phase 3 - Make sure the package works and send a PR

• Run the full suite of Atom tests with script/test and verify that they pass locally
• If any tests fail, investigate and fix them then commit the fixes to your branch
• Create a PR against atom/atom to verify that CI passes
• Once CI is clean and the PR is approved, merge it

Phase 4 - Archive the encoding-selector repo

• Move all existing issues from the encoding-selector repo to atom/atom using github-issue-mover or a script
• Add a label titled packages/encoding-selector to the migrated issues so that they can be found easily
• Edit the original package repo's README.md (don't commit directly) to point contributors to the code's new home in atom/atom. You can use the following text:

  ### This package is now a part of the [core Atom repository](https://github.com/atom/atom/tree/master/packages/encoding-selector), please direct all issues and pull requests there in the future!
  
  ---
  

• Send a PR to the encoding-selector repository with the updated README.md so that watchers will be notified of the change. You do not need to wait for reviews, it can be merged immediately.
• Archive the package repository by going to its Settings page, scroll down to the "Danger Zone" section and click "Archive this Repository"

Migration is now complete! 🎉