Enable Go module support across all commands

[tpounds]

Misc. cleanup to dedupe GO111MODULE configurations and enable Go modules across all make targets.

[tariq1890]

Is this required? Go 1.13 has this on by default.

[tpounds]

@tariq1890 Go 1.13 defaults it to auto which implies module-awareness by default (see: https://golang.org/doc/go1.13#modules). However, the primary reason for this change is to consolidate all of the places it was being set and to force consistent module on behavior for both Go 1.12 and 1.13 builds in CI (see: https://github.com/golangci/golangci-lint/blob/master/.travis.yml#L2). FWIW, I also tested this change against Go 1.11 locally and it seemed to work fine. It may be possible to re-enable Go 1.11 in CI if there aren't any internal Go runtime/interface issues to be wary of. @jirfag Thoughts?

[jirfag]

thank you, I agree, good change