Migrate to Go Modules

[marwan-at-work]

This PR is generated by a tool I wrote that helps migrate Go packages that are tagged +2 to Semantic Import Versioning.

Note that this change should be backwards compatible and can still be imported without the /vX suffix when Go Modules is off.

[dgrijalva]

Thanks for this. This looks like a backwards incompatible change. I need to look into this a little bit to ensure we don't violate semver, but it looks like it's time for this to land.

[marwan-at-work]

@dgrijalva yes the recommendation from the Go team is that this should be a new major version upgrade :)

[danqing]

any news on this @dgrijalva ? would love to see this merged

[marwan-at-work]

Since the current release is v3 this PR would not work well with modules, I made a change to make it v4 instead of v3.

To be clear, adopting modules is considered a breaking change if your library's major version is >= 2 (jwt-go is 3).

[BryceDFisher]

I would definitely like to see this integrated into the v4 release.