plugin: Switch internal Terraform rules to the bundled plugin

[wata727]

Fixes #1352
Fixes #1056
See https://github.com/terraform-linters/tflint-ruleset-terraform

This PR cuts out the Terraform rules built directly into TFLint into a plugin for historical reasons. The plugin is bundled in the TFLint core as a bundled plugin so that if the plugin is not explicitly enabled, it will be automatically enabled. This allows end-users to use Terraform rules almost the same as before.

All rules are available if the plugin is enabled, but due to how this bundled plugin works, the "recommended" preset is automatically set when enabled automatically. This means that the following configuration is automatically inserted:

plugin "terraform" {
  enabled = true
  preset  = "recommended"
}

https://github.com/terraform-linters/tflint-ruleset-terraform/blob/1891988f8797be2006a30702505d7e00774ad3c0/docs/configuration.md

Please note that the "recommended" preset is different from the current default rules. The following rules are added to the rules enabled by default:

• terraform_deprecated_index
• terraform_required_providers
• terraform_required_version
• terraform_typed_variables
• terraform_unused_declarations

This behavior can be changed by explicitly declaring a "terraform" plugin block. For example, the following declaration will enable all rules instead of the "recommended".

plugin "terraform" {
  enabled = true
}

As with any plugin, you can enable a different version of the plugin than the bundled plugin by declaring the version and source. In this case, you need to run tflint --init.

plugin "terraform" {
  enabled = true

  version = "0.1.0"
  source  = "github.com/terraform-linters/tflint-ruleset-terraform"
}

TODO

• Move cli_test.go to integration tests
• Add integration tests for the bundled plugin
• Add unit tests for the bundled plugin config
• Concerns about terraform_required_version rules running in non-Terraform workspaces
• Revise documentations