x/playground: Format button produces a spurious 'invalid module version' error for commit hashes in go.mod

[bcmills]

go subcommands such as go mod tidy, go test, and go build automatically resolve invalid versions found in the go.mod file to valid ones.

However, the Playground's Format button does not; instead, it emits an error message like:

go.mod:5: invalid module version "1f3472d942ba824034fb77cab6a6cfc1bc8a2c3c": version must be of the form v1.2.3

This is probably due to calling out to a golang.org/x/mod library instead of running the go command directly. (See also #32955 (comment) and #32614.)

CC @toothrot @dmitshur @ysmolsky

[bcmills]

Example: https://play.golang.org/p/yMyG925ovLb

Format produces an error, but Run runs the binary without error.