[Monorepo] index has to be set twice

[BlechbiegerBender]

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

1. Setup monorepo structure as in 🚀 Example repository
2. Setup dependencies like described in pdm docu
   project/pyproject.toml:

[tool.pdm.dev-dependencies]
dev = [
    "-e file:///${PROJECT_ROOT}/packages/foo-core",
    "-e file:///${PROJECT_ROOT}/packages/foo-cli",
    "-e file:///${PROJECT_ROOT}/packages/foo-app",
]

3. Add company index on repo root level
   project/pyproject.toml:

[[tool.pdm.source]]
name = "private"
url = <company-url>
verify_ssl = true

4. Set up pdm.toml on top level to ignore pypi.org in order to be able to work on machines that are blocked from Internet access by a firewall

[pypi]
ignore_stored_index = true

6. Build sub package:

pdm build -p .\packages\<subpackage> -v

Actual behavior

PDM fails to build package if index is only set in repo root pyproject.toml

pdm build -p .\packages\<subpackage> -v
Building sdist...
pdm.termui: Preparing environment(Isolated mode) for PEP 517 build...
pdm.termui: ======== Start resolving requirements ========
pdm.termui:   pdm-backend
pdm.termui:   python>=3.10.11,<3.10.12
pdm.termui:   Adding requirement pdm-backend
[PdmUsageError]: You must specify at least one index in pyproject.toml or config.
The 'pypi.ignore_stored_index' config value is True

If index is set in sub package packages\<subpackage>\pyproject.toml too, PDM successfully builds package.

Expected behavior

Maybe my expectation is not correct, but I would expect that the set index is used for sub packages as well.

Environment Information

# Paste the output of `pdm info && pdm info --env` below:
PDM version:
  2.12.3
Python Interpreter:
  C:\git\pdm-example-monorepo\.venv\Scripts\python.exe (3.10)
Project Root:
  C:/git/pdm-example-monorepo
Local Packages:

{
  "implementation_name": "cpython",
  "implementation_version": "3.10.11",
  "os_name": "nt",
  "platform_machine": "AMD64",
  "platform_release": "10",
  "platform_system": "Windows",
  "platform_version": "10.0.19045",
  "python_full_version": "3.10.11",
  "platform_python_implementation": "CPython",
  "python_version": "3.10",
  "sys_platform": "win32"
}

[frostming]

Currently, there is no subordinate relationship between the root package and the sub packages, they are treated independently. The UI design relies on the support of workspace(#1505 )