added -provider cli flag for tfschema show

[shlomimatichin]

referencing env0/terratag#17

look at the example below: google_billing_budget resource
comes from google-beta provider. previously, code assumed
the provider name is the first part by underscore, e.g., google.

data "google_billing_account" "account" {
  provider = google-beta
  billing_account = "000000-0000000-0000000-000000"
}

resource "google_billing_budget" "budget" {
  provider = google-beta
  billing_account = data.google_billing_account.account.id
  display_name = "Example Billing Budget"

  budget_filter {
    projects = ["projects/my-project-name"]
    credit_types_treatment = "EXCLUDE_ALL_CREDITS"
    services = ["services/24E6-581D-38E5"] # Bigquery
  }

  amount {
    specified_amount {
      currency_code = "USD"
      units = "100000"
    }
  }

  threshold_rules {
    threshold_percent = 0.5
  }
  threshold_rules {
    threshold_percent = 0.9
    spend_basis = "FORECASTED_SPEND"
  }
}

[minamijoyo]

@shlomimatichin Good point! It's true that the google-beta provider has an irregular naming.

However, not only google and google-beta provider use google_ prefix, strictly speaking, but also any forks such as registry.terraform.io/foo/google can use google_ because Terraform v0.13 introduced a hierarchical namespace for providers. This means that we cannot strictly detect provider binary only from a resource name.

In this case, I think it's better to allow users to pass a fully qualified provider source rather than a local provider name.
https://www.terraform.io/docs/configuration/provider-requirements.html

$ tfschema resource show -source registry.terraform.io/hashicorp/google-beta google_billing_budget

If we omit the registry hostname, it's reasonable to implicitly assume registry.terraform.io by default, just like Terraform. So a shorthand version can be the following:

$ tfschema resource show -source hashicorp/google-beta google_billing_budget