Motivation for this change

Terraform has undergone some major changes which makes it unusable with v20.09; mainly a change that prevents using symlinks to identify plugins. Terraform implemented this change to prevent infinite loops when trying to search for plugins. However, this breaks terraform since the nix-store relies heavily on symlinks.

This commit backports the latest changes from master to ensure terraform runs as expected on v20.09.

Things done

In a fully sync'd repository, starting from branch release-20.09, I ran the following:

git checkout master -- pkgs/applications/networking/cluster/terraform
git checkout master -- pkgs/applications/networking/cluster/terraform-providers
git commit -am "Backported latest changes from master"

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.