Add /v4 to references to dafny-lang/DafnyRuntimeGo

[robin-aws]

Description

We fixed the github.com/dafny-lang/DafnyRuntimeGo repository by adding a copy under v4/, but need to update all of the references to that path in Dafny itself.

How has this been tested?

Existing tests of Go using --go-module-name. Also added an explicit test against the published runtime, since the existing ones use replace directives to test against the local copy in DafnyRuntimeGo-gomod instead.

I also found for this test to work, I had to push a new tag on github.com/dafny-lang/DafnyRuntimeGo, since the checksum didn't match on the existing v4.7.0 tag. I suspect we moved the tag at some point when debugging the issues, which does not work since the Go checksum repository can't be updated. I went with v4.8.0 just in case it still didn't work and we needed to make extra changes and then tag with v4.8.1.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[ShubhamChaturvedi7]

We should have a separate integration tests which run against the published version. DafnyRuntimeGo-gomod was added because we didn't have a published library back then. We should run local tests against a copy which is generated by Dafny, and afaik DafnyRuntimeGo-gomod is static.

[robin-aws]

We should have a separate integration tests which run against the published version. DafnyRuntimeGo-gomod was added because we didn't have a published library back then. We should run local tests against a copy which is generated by Dafny, and afaik DafnyRuntimeGo-gomod is static.

I realized this was worth doing and not hard, the only caveat was potentially needing to publish a new version of the runtime before a new version of Dafny itself, but I think that's worth it. Added a test and updated the PR description.