Remove vendoring

[radeksimko]

This PR removes vendoring, as inspired by Terraform and motivated by the fact that terraform-exec currently depends on go-getter which in turn brings plenty of transitive dependencies, so this should make #271 much smaller.

[paultyng]

is this not set by default?

[radeksimko]

The default (according to go env) is https://proxy.golang.org,direct and so the idea here is that by excluding direct we can effectively protect ourselves against the "leftpad situation" where the source would suddenly disappear by making sure that all dependencies are always cached before being added.

[paultyng]

LGTM, one comment, also I wanted to inquire internally again to see if we have a proxy or not.

[radeksimko]

I wanted to inquire internally again to see if we have a proxy or not.

Sure, I'm happy to add it to the config once we confirm we have one, but I assume it's not a blocker.

[ghost]

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