Tell PDM to use private index only for selected packages

[baggiponte]

Hi! This is a minor thing, I guess. Where I work, we are using a private index to store private packages, and the private index is the primary one (the other is pypi). This means that when I pdm add/install/update, every package is looked for in the private index first, then in pypi if it's not there. Since we plan to have just a handful of packages in the private index, I was wondering if there was a way to tell the package manager to look at the private index only for specific packages. Thanks!

[frostming]

May be duplicated with #1645