New issue by freddylist on void-packages repository

https://github.com/void-linux/void-packages/issues/39900

Description:
Something similar has been discussed before in #19830, but I thought I would bring it up again since it's been 2 years and I think it could be nice to have something like Gentoo's overlays or Arch's AUR (a central repository that is more "liberal" in what is accepted) or something for packages that cannot be accepted to the main repositories, such as
 - browsers (brave, librewolf, icecat, etc.)
 - bleeding-edge software (git master branch, unreleased software)
 - binary distributions of software (large software like browsers, software with large outdated build dependencies e.g. [Zig](https://github.com/void-linux/void-packages/pull/37164#issuecomment-1128926851))
 - packages that the original packager isn't intending to maintain thoroughly
 - other minor things (themes and such)
 - and so on...

Right now, the ad-hoc way of building packages distributed outside of `void-packages` is to copy the templates into the `srcpkgs/` directory and building with `xbps-src` from there. This is fine for managing a couple of externally distributed packages, but can quickly become a hassle. One could maybe write a script to help manage packages like this, but this is very error-prone.

`xbps-src` expects all symlinks in the `srcpkgs/` directory to be subpackages, so we can't just symlink packages into `srcpkgs/` or use a symlink farm manager like GNU Stow, which might otherwise be a slightly better ad-hoc solution.

I also randomly tried to symlink to `xbps-src` and its dependencies from an external template repository, and it seems that it won't work without some patching. Copying `xbps-src` and its dependencies also doesn't seem to work without also copying a couple of packages from `void-packages/srcpkgs/`, such as `xbps-triggers` and `base-files`, which isn't ideal.

With all that said, I think it would be good to have a Void-endorsed way to create user template repositories/packages documented somewhere. It almost goes without saying that there are inherent security risks with using these user template repositories, and that should of course be documented as well. So, does there currently exist better way to create user repositories/packages, or must `xbps-src` be updated to support this?