Add tooling to consistently format Go imports

[fasaxc]

Description

• Split make fix into make fix-changed and make fix-all
  • Use fix-all in CI, it runs no slower on semaphore than the old version.
  • fix-changed should be faster when run locally.
• make fix-changed only formats files that have changed on this branch.
  • The parent branch is guessed based on distance to the merge base with master and release branches.
  • The remote name is auto-detected from metadata.mk.
• Remove check-fmt target in favour of running a format pass andchecking for dirty in CI (hard to do check-fmt now that two commands need to be run in sequence).

Related issues/PRs

Todos

• [ ] Tests
• [ ] Documentation
• [ ] Release note

Release Note

TBD

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[nelljerram]

@fasaxc While you're in the area, I wonder if you have any thoughts about https://github.com/projectcalico/calico/pull/9277 - i.e. about why our make fix doesn't make the changes that are proposed in that PR.

[fasaxc]

We use goimports, which is a superset of gofmt, but:

• it's likely pinned to a specific version
• perhaps we don't specify -s for "simplify".