This new package aims to encapsulate all of our interactions with
go-getter to fetch remote module packages, to ensure that the rest of
Terraform will only use the small subset of go-getter functionality that
our modern module installer uses.
In older versions of Terraform, go-getter was the entire implementation
of module installation, but along the way we found that several aspects of
its design are poor fit for Terraform's needs, and so now we're using it
as just an implementation detail of Terraform's handling of remote module
packages only, hiding it behind this wrapper API which exposes only
the services that our module installer needs.
This new package isn't actually used yet, but in a later commit we will
change all of the other callers to go-getter to only work indirectly
through this package, so that this will be the only package that actually
imports the go-getter packages.
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