Reference validation for remaining parts of the language

[radeksimko]

Context

Reference validation was introduced as part of #1357 and further refactored in #1346

In short, this produces the following diagnostics

We have decided to implement only validation of variables (var.* references) and local values (local.*). All other references are currently excluded from validation.

• data sources (data.*)
• resources
• modules
• "extensions"
  • self.* references
  • each.*
  • count.index
• provider aliases (e.g. aws.east in a provider attribute of a resource)

Most of these were initially excluded because they are fully or in part dependent on us knowing full schema where we more often may not. e.g. user may be using provider supercloud and has references to resources such as supercloud_instance.foo.public_ip and because we may not know the schema of supercloud we cannot tell whether the full reference is valid or not, i.e. whether public_ip exists on supercloud_instance.

Ideally we should be able to at least validate parts of the reference, e.g. tell whether supercloud_instance.foo is valid, i.e. whether resource "supercloud_instance" "foo" was declared or not, but this all requires a more complex solution we have not implemented yet.

Proposal

• validations: Enable reference validation of locals #1381
• Enable reference validation for resources (e.g. aws_instance.* or resource.aws_instance.*)
• Enable reference validation for data sources (data.*)
• Enable reference validation for module outputs (module.*)
• Enable reference validation for provider aliases (e.g. aws.foo-alias in provider attribute)
• Enable reference validation concerning language extensions
  • self.*
  • each.*
  • count.index
  • references to an item being looped over in dynamic block's content