rename packet_* to metal_* and rename provider to equinix/terraform-provider-equinix-metal

[displague]

The current default branch for the Packet provider is master.

This PR will apply to a new default branch, main, which will be used as a new Equinix Metal provider at equinix/terraform-provider-equinix-metal.

See https://github.blog/changelog/2020-08-26-set-the-default-branch-for-newly-created-repositories/ for more on the master and main branch names.

This repository will be moved to equinix/terraform-provider-equinix-metal and will then be forked back to packethost/terraform-provider-packet. The packethost/terraform-provider-packet fork should continue to feed into the TF Registry listing for Packet https://registry.terraform.io/providers/packethost/packet/latest (at the existing v3.1.0), a new registry listing will be created for equinix/terraform-provider-equinix-metal at https://registry.terraform.io/namespaces/equinix, this will be v4.0.0.

The main branch will be removed from the packethost repo, and master will be made the default on packethost (as it is now). This will allow for the Packethost provider to continue working while avoiding the confusion that would come from having two branches that represent different providers.

The master branch on packethost will receive additional commits to introduce deprecation notices.

Migration notes will reside in the Equinix Metal README.md.

The packet_ resources have all been renamed to metal_.

Users will face challenges in migrating to this new provider. It should be possible by doing the following:

• terraform plan and look for no changes, resolve changes first
• backup the existing configuration directory and state
• rename the packet_ references in the module to metal_
• rename packethost references in the module to equinix
• do the same 2 replacements within the tfstate files
• terraform init --upgrade to fetch the equinix-metal provider
• terraform plan and look for no changes, if the plan is clean it should be safe to apply

These changes were made mechanically with an order and search filters that prevented renaming things that did not need renaming or should not be renamed.

Fixes #279

Caveat on Issue links

GitHub forwarding will not take users from packethost/t-p-p -> equinix/t-p-e-m, because the packethost fork will reside at packhoet/t-p-p. This will cause external links to GH issues and pull-requests on t-p-p to 404. Older links to terraform-providers/terraform-provider-packet, however, will successfully redirect to equinix/t-p-e-m. Maintaining these issue and PR links is not a primary concern.

[displague]

This PR is ready and the tests are passing (as well as can be expected with jitter).

I'll be meeting early next week to discuss any logistical issues with the new repository scheme.

Barring any complications, this should be ready to merge and migrate after that meeting.

[displague]

I'm getting some boxes checked on this rename. With the US holidays this week, this will not land any sooner than 2020-11-30.