Draft: Bug 40849: Use `go mod vendor` to fetch go dependencies

Merge Info

• Related Issues

  • Closes #40849 (closed) #33953 (closed) #40673 (closed)

• Backport Timeline

  • Immediate - patchsets for critical bug fixes or other major blocker (e.g. fixes for a 0-day exploit) OR patchsets with trivial changes which do not need testing (e.g. fixes for typos or fixes easily verified in a local developer build)
  • Next Minor Stable Release - patchset that needs to be verified in nightly before backport
  • Eventually - patchset that needs to be verified in alpha before backport
  • No Backport - patchset for the next major stable

• Issue Tracking

  • Link resolved issues with appropriate Release Prep issue for changelog generation

Change Description

Update conjure, webtunnel, obfs4 and snowflake to use go mod vendor to fetch dependencies.

We still have some go module projects, for dependencies of projects/ncprop279/config, which doesn't have a go.sum file yet.

I think someone from @tpo/anti-censorship team should also be reviewing this.