migrate away from git.torproject.org

The plan is to deprecate git.tpo (tpo/tpa/team#40472 (closed)). Let's migrate away from it.

Needed tasks:

• decide a commit signing workflow that works for us (#8)
• migrate go packages to use gitlab.tpo instead of git.tpo as their module name.
  • https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/conjure
  • https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/meek
  • https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake
• move goptlib to gitlab.
• make sure TB, guardian project and others use the new repos