command/version: Report the current platform

[apparentlymart]

Along with all of the other information we previously reported in the terraform version output, we'll now include the name of the current platform as our provider installation mechanisms represent it.

This is addressing a long-standing minor annoyance where we sometimes can't tell from an incomplete bug report which platform Terraform was running on, and incomplete bug reporters do tend to at least include the requested terraform version output even if they don't also include the requested full trace log.

However, what motivated doing this now is that anyone building a provider registry or mirror needs to have some awareness of these platform identifiers which have been, until v0.13, mostly an implementation detail. This additional information is a small thing we can do to help registry builders (or folks publishing providers for more esoteric platforms) find out what the platform identifier ought to be for each of the platforms they aim to support, even if some of them are platforms which the Go compiler allows but which HashiCorp doesn't officially support.

The new information is on a line of its own in the output as a pragmatic way to avoid breaking anyone who might be using something like $(terraform version | head -n1) to print a brief Terraform version identifier into some logs. That's not an interface we officially support for machine consumption, but it's easy to avoid breaking it here and so we won't do so. (If you do want an interface for machine consumption, use terraform version -json instead.)

Terraform v0.15.0-dev
on linux_amd64
+ provider registry.terraform.io/apparentlymart/testing v0.0.2
+ provider registry.terraform.io/hashicorp/azurerm v2.1.0
+ provider registry.terraform.io/hashicorp/null v3.0.0
+ provider registry.terraform.io/newrelic/newrelic v2.1.2

{
  "terraform_version": "0.15.0-dev",
  "terraform_revision": "",
  "platform": "linux_amd64",
  "provider_selections": {
    "registry.terraform.io/apparentlymart/testing": "0.0.2",
    "registry.terraform.io/hashicorp/azurerm": "2.1.0",
    "registry.terraform.io/hashicorp/null": "3.0.0",
    "registry.terraform.io/newrelic/newrelic": "2.1.2"
  },
  "terraform_outdated": false
}

---

While I was working on this I also noticed that we forgot to update the version package to report the current development as being 0.15.0, so there's another commit in this PR to update that.

[codecov]

Codecov Report

Merging #26975 (76cd9eb) into master (b8d448d) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

Impacted Files	Coverage Δ
commands.go	0.67% <0.00%> (-0.01%)	⬇️
command/version.go	69.73% <100.00%> (+0.81%)	⬆️
terraform/node_resource_plan.go	95.28% <0.00%> (-1.89%)	⬇️
terraform/evaluate.go	52.89% <0.00%> (-0.42%)	⬇️

[ghost]

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