Add schema keywords in lockfile

[skshetry]

This is a 2.0 change introduced in iterative/dvc#5222.

Though there wasn't stages keyword in dvc.lock file in 1.0 version, I found a couple of files that had them. But, schema and stages were introduced at the same time. schema is just set to 2.0 when DVC v2.0 tries to dump into the lockfile. v1.0 lockfiles are migrated at the same time if needed, to v2.0. stages just wrap the pre-existing structure of lock entries.

In 1.0

echo-hello:
  cmd: echo hello world
echo-hi:
  cmd: echo hi

In 2.0

schema: '2.0'
stages:
  echo-hello:
    cmd: echo hello world
  echo-hi:
    cmd: echo hi

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[jorgeorpinel]

Actually, they are OK in the latter example but I'll remove it from here to simplify. I'll make a follow-up PR to explain the size (and nfiles?) fields in dvc.lock.

Suggested change

size: 2712300

UPDATE: See #2139

[jorgeorpinel]

Thanks @skshetry !

[jorgeorpinel]

p.s. we may also need to follow-up with an update to all other dvc.lock samples throughout docs...

UPDATE: See #2139