This new command is intended to make it easy to create or update a mirror
directory containing suitable providers for the current configuration,
producing a layout that is appropriate both for a filesystem mirror or,
if copied into the document root of an HTTP server, a network mirror.
This initial version is not customizable aside from being able to select
multiple platforms to install packages for.
Future iterations of this could include commands to turn the JSON index
generation on and off, or to instruct it to produce the unpacked directory
layout instead of the packed directory layout as it currently does. Both
of those options would make the generated directory unsuitable to be
a network mirror, but it would still work as a filesystem mirror.
In the long run this will hopefully form part of a replacement workflow to
terraform-bundle as a way to put copies of providers somewhere so we don't
need to re-download them every time, but some other changes will be needed
outside of just this command before that'd be true, such as adding support
for network and/or filesystem mirrors in Terraform Enterprise.
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