fix(go)!: Don't fallback if GOPROXY used (#12407)

Current implementation tries to use GOPROXY and falls back to Renovate fetching mechanism if no releases found.

The new one is switches to GOPROXY implementaiton when environment variable is set and doesn't fallback.
However, when direct keyword is used, it will use Renovate-native mechanism that fetches directly from GitHub, etc.
When off keyword is encountered or no URLs left, we're done with no releases (i.e. no fallback to Renovate-native mechanism).

BREAKING CHANGE: Go modules lookups will now no longer fallback to Renovate native lookups if GOPROXY is configured and without "direct" explicitly configured.