Add explicit support for ** in glob

[cbednarski]

Glob in Go doesn’t actually work the way it’s tested here:

Include: []string{"build/**/*"},

This test passes in *nix because (I think) the underlying shell takes over and expands the path. However, this doesn't work on Windows, and so is not portable. This is technically a bug in the test but people may be using this in the wild... Do we want to fix it by adding ** support for Windows?

[mitchellh]

Talked about this in Slack but will add context here:

https://hashicorp.slack.com/archives/packer/p1433819316000060

[cbednarski]

To recap in the issue, we do want to support this on Windows. Marked as bug.