Remove or replace the configdir library

[na--]

We currently use https://github.com/shibukawa/configdir to get the user level configuration folder (e.g. ~/.config/loadimpact/k6) in a cross-platform way. But it's a somewhat poor fit, and since it directly manipulates files via the os package, it's not compatible with https://github.com/spf13/afero and we can't use the majority of its functions directly in any testable way.

So as part of #935, I've mostly refactored it away, and we use it in only a single place to get the default location for the config.json file. So we can either replace it with something else, or we can easily write our own version in a few lines (basically, we only need to replicate globalSettingFolder from that library).

[na--]

Seems like we just have to wait for Go 1.13 for this: golang/go#29960

[na--]

https://golang.org/pkg/os/#UserConfigDir