earlydecoder: Decode unknown modules & provider requirements

[radeksimko]

Currently we decode all module calls into a map, where the key is the module reference name and ignore any module calls which do not have a reference name + override modules of the same name.

module "" {
  source = ""
}

module "" {
  source = ""
}

Similarly we decode required providers into a map, keyed off provider local name and override any requirements with the same name.

terraform {
  required_providers {
    name = {
      source = ""
    }
    name = {
      source = ""
    }
  }
}

Map of course makes lookups much easier, but it makes it practically impossible to track unknown modules and provide some IntelliSense within those.

For module calls in particular map is a reasonable structure for hashicorp/terraform-ls#598 and hashicorp/terraform-ls#725 but unnamed modules may also be helpful to parse in the context of hashicorp/terraform-ls#869

Similarly hashicorp/terraform-ls#871 may benefit from decoded requirements of the same name.

Originally posted by @radeksimko in #106 (comment)