cmd: Deprecate CLI arguments

[wata727]

See also #1612 #1638

Previously, TFLint accepted files and directories as arguments.

$ tflint main.tf
// print only main.tf issues from the directory

$ tflint modules/production
// inspect modules/production directory

This is a natural interface as a linter. However, as a Terraform language static analyzer, there are some issues.

First, the Terraform language must evaluate modules (directories), so it cannot evaluate a single file. This is clear if you imagine the example of evaluating an expression like var.foo in the presence of variables.tf and main.tf respectively.

Second, in the Terraform language, the current directory is included in the semantics. Imagine an example like file("./policy.json"). From this you can see that tflint modules/production and cd modules/production; tflint give different results.

Terraform has dropped support for arguments and introduced the -chdir flag to address the second issue. hashicorp/terraform#26087. TFLint focuses on Terraform language compatibility, so we try to make the interface as uniform and unambiguous as possible.

Recently, TFLint introduced the --chdir and --filter flags. It can replace the arguments as follows:

$ tflint --filter=main.tf
// replacement of "tflint main.tf"

$ tflint --chdir=modules/production
// replacement of "tflint modules/production"

$ tflint --chdir=modules/production --filter=main.tf
// replacement of "tflint modules/production/main.tf"

(As a side note, for the same reasons as Terraform, it is recommended that these options be placed at the beginning of all flags and joined with =)

This PR changes it to print a warning to stderr like the following if the arguments are still in use:

$ tflint main.tf
WARNING: "tflint FILE/DIR" is deprecated and will error in a future version. Use --chdir or --filter instead.

$ tflint --version modules/production
WARNING: Arguments are not used in version mode and will error in a future version. Use --chdir instead.

$ tflint --langserver modules/production
WARNING: Arguments are not used in language server mode and will error in a future version.

$ tflint --init modules/production
WARNING: Arguments are not used in init mode and will error in a future version. Use --chdir instead.

We are planning to drop support for arguments in v0.47. It will work as before in v0.46, but we recommend migrating early.