Terragrunt (Cache server) fails when absolute URLs used in registry's self-discovery (`modules.v1`)

[t3hG04T]

Describe the bug

When using Cache Server and trying to pull a sub-module from a Terraform registry that uses absolute URLs in it's self discovery the module will fail to retrieve with a message:

  │ Error: Error accessing remote module registry
  │ 
  │   on my-module.tf line 1:
  │    1: module "my-module" {
  │ 
  │ Failed to retrieve available versions for module "my-module"
  │ (my-module.tf:1) from private.registry.url: the request failed after
  │ 2 attempts, please try again later: Get
  │ "https://<hostname><absolute-url>/registry/my-module/aws/versions":
  │ dial tcp: lookup <hostname>https on <IP>:<PORT>: no such
  │ host.

Steps To Reproduce

1. Use a registry that uses absolute paths in it's self-discovery, e.g.:

$ curl https://packages.syncron.team/.well-known/terraform.json
{
  "modules.v1" : "https://<hostname>/somepath/modules/",
}

2. Pull the module within a .tf file:

module "my-module" {
  source = "<hostname>/registry/my-module/aws"
  version = "= 1.0.0"
}

3. Try to run terragrunt plan
4. Observe the error

Expected behavior

Terragrunt pulls the module

Versions

• Terragrunt version: 0.93.11
• OpenTofu/Terraform version: 1.8.5
• Environment details (Ubuntu 20.04, Windows 10, etc.): Ubuntu 24.04

Additional information

I've traced the root of the problem to https://github.com/gruntwork-io/terragrunt/blob/main/internal/providercache/provider_cache.go#L322

It seems that the Cache Server is naively assuming relative URLs without double checking (and DiscoveryURL is also just a container for the contents present in the file).