Add go.mod to go runtime module

[Joakker]

Hopefully fixes #3175

[parrt]

Weird. still failing: go.mod not found still. Must be in wrong dir? Our test does something weird when running go?

[Joakker]

I'm inclined to believe it's the CIitself that's the problem, because on circle-ci, the go runtime tests are successful.

I have the suspicion that it's pulling from the github repo instead of the forked copy with the module file. Since go imports by url (import "github.com/antlr/antlr4/runtime/Go/antlr"), and the repo that the url is pointing to doesn't have the module file, it's not gonna find it like that.

So yeah, I think it's the test that is the problem

[parrt]

Hmm...ok, so I'll incorporate and retest.

[Joakker]

Ok, I don't know why it keeps failing. We might have to rewrite the test

[Joakker]

Actually, I think I do. The go proxy (which is used by the go tool to download packages from the internet), is a bit slow to update, especially for pseudo-releases, which would be the case here. That can be attested by the fact that the documentation page still says that it lacks a go.mod file @parrt

[parrt]

Ok, it now seems to think we have one: checked Valid go.mod file :) Still seems to be failing though I think

[Joakker]

At least it's progress :D