[WIP] Fix duplicate struct registration with v.mod

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

fix v.mod, c_buid, parser, or ast.
test suggestion

import os

fn test_duplicate_struct_registration_with_vmod() {
	root := os.temp_dir() + '/vmod_struct_dup_test'
	os.rmdir_all(root) or {}
	os.mkdir_all(root + '/hexagonal/application') or { panic(err) }
	os.write_file(root + '/v.mod', 'Module {\nname: "vanilla3"\n}') or { panic(err) }
	os.write_file(root + '/hexagonal/application/auth_usecase.v', 'module application\n\npub struct AuthUseCase {}\n') or {
		panic(err)
	}
	os.write_file(root + '/hexagonal/main.v', 'module main\n\nimport application\n\nfn main() { _ := application.AuthUseCase{} }\n') or {
		panic(err)
	}

	// Simulate the real project structure and import resolution
	// Run from the root so v.mod is picked up, and import path is correct
	old_dir := os.getwd()
	os.chdir(root) or { panic(err) }
	res := os.execute('v run hexagonal')
	os.chdir(old_dir) or { panic(err) }
	dump(res.output)

	os.rmdir_all(root) or {}
}

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