Adds the ability to run tasks from the root package.json file.

Implementation notes that will need to be documented:

• Tasks run from the root are opt-in. That is, turbo.jsonmust have an entry for _root#<task> to enable running that task at the root.
• _root is available in filters, so turbo run my-task --filter=_root will run _root#my-task.
• If an entry exists in turbo.json for _root#task, then turbo run task will also run that task for the root.
• There is some best-effort cycle detection in place. If the root package.json has "build": "turbo run build", then even after opting in, turbo run build --filter=_root will produce an error. This cycle detection can be circumvented with aliases or other indirection used to run turbo.
• For the purposes of changed files, any changed file in a repo that doesn't belong to another package is considered to belong to the root.
  • Note that this could cause frequent runs of root tasks if they are triggered from git changes. With inputs specified, they should hit cache if nothing relevant has changed.

Additional changes:

• e2e tests now include a root task
• a package task is now sufficient to mark a particular task as runnable by turbo. e.g. if you defined foo#my-task but not my-task in turbo.json, previously running turbo run my-task would be an error, now it will run foo#my-task.

Fixes #845