Closed issue by ericonr on void-packages repository

https://github.com/void-linux/void-packages/issues/31526

Description:
Writing a template to depend on `zstd-devel` (removed in d481489dae9449f5212f4e2b5fab74e858962242) should fail because `srcpkgs/zstd-devel/template` doesn't exist. Instead, if it's still available in remote repositories because it hasn't been cleaned up, it will be enough to satisfy the dependency.

An example from adding `zstd-devel` to `depends` for `kicad-library`.

```
   [target] zstd-devel: found (https://alpha.de.repo.voidlinux.org/current/musl)
   [runtime] kicad-footprints-5.1.10_1: found (https://alpha.de.repo.voidlinux.org/current/musl)
   [runtime] kicad-packages3D-5.1.10_1: found (https://alpha.de.repo.voidlinux.org/current/musl)
   [runtime] kicad-symbols-5.1.10_1: found (https://alpha.de.repo.voidlinux.org/current/musl)
   [runtime] kicad-templates-5.1.10_1: found (https://alpha.de.repo.voidlinux.org/current/musl)
```

This can lead to issues such as 5898b6cbe0efcf6c129799a0993943fc7128f1a7 (fixed in d0ae88846bf833a5e1171dff6dd44a65c4891466), where a package can be made to depend on another one that's no longer available.

When fixing this, care should be taken to not break virtual packages or others depending on the `provides=` mechanism (though arguably packages which are `provides`d by something are valid only in `depends`).