fix: Fetch only teams required for environments (#17)

Saves fetching all of the teams in the org for every repository.
Especially wasted API calls when there are no environments being
defined.

I've never used the `environments` input before, but the following
*works* with this new code and fails with the old code. So... Was the
old code broken as well?

```
module "blah {
...
...
  environments = {
    production = {
      reviewer_teams = [
        "team-1",
        "team-2"
      ]
      reviewer_users = [
        "user-1"
      ]
      deployment_branch_policy = {
        protected_branches = true
        custom_branch_policies = false
      }
    }
  }
}

Author: gaima8

README.md

@@ -120,7 +120,7 @@ See [variables.tf] and [examples/] for details and use-cases.
 | [github_repository_webhook.repository_webhook](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/repository_webhook) | resource |
 | [github_team_repository.team_repository](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/team_repository) | resource |
 | [github_team_repository.team_repository_by_slug](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/team_repository) | resource |
-| [github_organization_teams.all](https://registry.terraform.io/providers/integrations/github/latest/docs/data-sources/organization_teams) | data source |
+| [github_team.reviewers](https://registry.terraform.io/providers/integrations/github/latest/docs/data-sources/team) | data source |
 | [github_user.user](https://registry.terraform.io/providers/integrations/github/latest/docs/data-sources/user) | data source |
 
 ### Inputs

data.tf

@@ -14,6 +14,27 @@ variable "environments" {
   default = {}
 }
 
+locals {
+  required_team_slugs = toset(flatten([
+    for env_name, env_config in var.environments : env_config.reviewer_teams
+  ]))
+  required_usernames = toset(flatten([
+    for env_name, env_config in var.environments : env_config.reviewer_users
+  ]))
+  team_ids_by_slug = { for t in data.github_team.reviewers : t.slug => t.id }
+  user_ids_by_name = { for u in data.github_user.user : u.username => u.id }
+}
+
+data "github_team" "reviewers" {
+  for_each = local.required_team_slugs
+  slug     = each.key
+}
+
+data "github_user" "user" {
+  for_each = local.required_usernames
+  username = each.key
+}
+
 resource "github_repository_environment" "this" {
   for_each            = var.environments
   repository          = github_repository.repository.name
@@ -24,7 +45,7 @@ resource "github_repository_environment" "this" {
   dynamic "reviewers" {
     for_each = length(each.value.reviewer_teams) > 0 || length(each.value.reviewer_users) > 0 ? [true] : []
     content {
-      teams = [for slug in each.value.reviewer_teams : try(local.team_ids_by_slug[slug], slug)]
+      teams = [for slug in each.value.reviewer_teams : local.team_ids_by_slug[slug]]
       users = [for username in each.value.reviewer_users : local.user_ids_by_name[username]]
     }
   }