Add module source address parsing

[dbanck]

This PR adds the parsing of module source addresses for registry modules. It exposes one new function:

• ParseRawModuleSourceRegistry

It's based on the internal/addrs/module_source.go code in terraform core with some modifications:

• Removed anything related to ModuleSourceRemote (e.g. GitHub or Bitbucket)
• Removed anything related to ModuleSourceLocal (e.g. ./example)
• Removed the go-getter dependency
  • Was required via internal/getmodules
  • Detection was required for ModuleSourceRemote
  • Inlined SourceDirSubdir

Example usage

moduleSourceRegistry, err := tfaddr.ParseRawModuleSourceRegistry(module.SourceAddr)
if err == nil {
	bodySchema.DocsLink = &schema.DocsLink{
		URL: fmt.Sprintf(
			`https://%s/modules/%s/%s`,
			moduleSourceRegistry.PackageAddr.Host.ForDisplay(),
			moduleSourceRegistry.PackageAddr.ForRegistryProtocol(),
			"latest",
		),
	}
}

Enables hashicorp/terraform-ls#877

[radeksimko]

This LGTM aside from one question/aspect:

Is there a particular reason for including ParseRawModuleSource, ModuleSourceLocal and the ModuleSource interface? The detection of local modules doesn't seem all that complicated and I think we could get away with it living either in terraform-ls, or potentially under terraform-schema: https://github.com/hashicorp/terraform-ls/blob/94aa1d8d66f28441fbf8e3273eefcc5554b9b817/internal/terraform/datadir/module_types.go#L61-L68

I guess there may be a convenience argument for including it, but the risk is that this gives anyone the expectation that the function can parse any valid module source address and I'd really prefer to avoid that scope creep, if possible - esp. because that allows us to avoid go-getter.

Maybe we'll re-scope the library as hashicorp/terraform-address at some point, but I'd prefer not to do that unless it's really necessary.

[dbanck]

Good call! I've removed the code related to local module parsing, and without the ModuleSource interface, usage of this library becomes much easier.

I updated the example and removed the type cast.

[radeksimko]

One more question + docs fix 😅

[radeksimko]

🚀