Rename fmt2 to fmt and lint2 to lint

[Eric-Arellano]

Closes #8351.

It's safe to make this change, even with us defaulting to --v2 for users, because we default to no V2 linters being enabled and the V2 goal no-ops in this case.

It's also safe to have both V1 and V2 linters run at the same time (mixed mode). They won't run at the same time, so there's no risk of formatters conflicting with each other.

In a followup, we can now deprecate V1 isort for the V2 implementation.

[benjyw]

🎉

[pierrechevalier83]

Really nice to see! 😄

[pierrechevalier83]

Maybe now it's not in the same file anymore, I would add a sentence to this comment to say: "when doing this, remember to change the help description for --only in rules/core/fmt.py." I could see us forgetting that otherwise.

[pierrechevalier83]

Note: This CI failure looks like a legit failure caused by this change: https://travis-ci.com/pantsbuild/pants/jobs/295575406

Exception message: Error computing value for --v1 in global scope (may also be from PANTS_* environment variables).
Caused by:
Traceback (most recent call last):
  File "/home/travis/.pex/code/9ee2a1c72fda298230e05408bcae28464e3b1aa3/pants/option/parser.py", line 285, in parse_args
    val = self._compute_value(dest, kwargs, flag_vals)
  File "/home/travis/.pex/code/9ee2a1c72fda298230e05408bcae28464e3b1aa3/pants/option/parser.py", line 733, in _compute_value
    dest, self._scope_str()
pants.option.errors.ParseError: Multiple cmd line flags specified for option v1 in global scope

[illicitonion]

Woo!