Running -format with GOFLAGS="-mod=vendor" fails if project has indirect dependencies

[nezorflame]

From what I can see, this happens:

1. gopkgs runs go list -mod=vendor -m -f={{.Path}}\;{{.Dir}} (notice that -mod=vendor is not in the code, but since we have GOFLAGS="-mod=vendor" it'll be acting if it was): https://github.com/uudashr/gopkgs/blob/master/gopkgs.go#L366
2. The output shows ALL of the packages, including the indirect ones
3. While walking through the list of packages in the loop https://github.com/uudashr/gopkgs/blob/master/gopkgs.go#L94 we eventually stumble upon an indirect dependency which wasn't pulled by go mod vendor, which in turn results in a folder which doesn't exist, hence causing the stat error.

I guess, there're two possible solutions:

1. Exclude indirect dependencies by using the {{if not .Indirect}}{{.Path}};{{.Dir}}{{end}} flag value for go list -f
2. Somehow make go mod vendor download the indirect dependencies as well (doubt that).

See details in microsoft/vscode-go#2800

[uudashr]

Should be fixed on #16