Allow the option to make index explicit when using `uv add`

[BaoNguyen6742]

Summary

When I add a package using specific index, packages that are installed later give "No solution found when resolving dependencies" error.

Step to reproduce

1. Running uv add paddlepaddle==3.0.0 --index https://www.paddlepaddle.org.cn/packages/stable/cpu/
   This will create the pyproject.toml file like this

[project]
name = "uv-index"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "paddlepaddle==3.0.0",
]


[tool.uv]
required-environments = [
    "sys_platform == 'win32'" # I add this since numpy 2.3.2 doesn't have a source distribution or wheel when I test on windows.
]

[[tool.uv.index]]
url = "https://www.paddlepaddle.org.cn/packages/stable/cpu/"

2. Run uv add matplotlib
   This will return the error

× No solution found when resolving dependencies for split (markers: python_full_version >= '3.12' and sys_platform
│ == 'win32'):
╰─▶ Because there are no versions of matplotlib and your project depends on matplotlib, we can conclude that your project's requirements are unsatisfiable.
help: If you want to add the package regardless of the failed resolution, provide the `--frozen` flag to skip locking and syncing.

There are currently 2 ways to fix this

1. Run uv add matplotlib with --index-strategy unsafe-first-match or --index-strategy unsafe-best-match
2. Edit the pyproject.toml in the tool.uv.index section to make the index explicit

   [[tool.uv.index]]
   url = "https://www.paddlepaddle.org.cn/packages/stable/cpu/"
   explicit = true
   

I hope there could be a way to make the index explicit right when you add it so it doesn't affect other package when installing

Example

No response