home-manager's generated completions shadow built-in completion scripts.
fish actually has logic to deal with this when the path ends with
/generated_completions, so let's take advantage of it.
Link: 47c773300a/src/autoload.rs (L421-L424)
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
