Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Removes hardcoded builtin package and its tools.

• What is the current behavior?

Certain tools like ctags, serial-discovery and mdns-discovery belong to the builtin package, that package doesn't exist in the Arduino package_index.json but is hardcoded in the arduino-cli.

Whenever we need to update one of these tools we must create a new release of the Arduino CLI, that's annoying for everyone.

• What is the new behavior?

builtin package is not hardcoded anymore, the Arduino CLI will assume the package is always present and install all its tools latest releases whenever an internal instance is initialized.

The builtin package will be added in the Arduino package_index.json.

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Before merging this we'll have to add the builtin package in the Arduino package_index.json.

See how to contribute