Add an option to set custom `$ZPLUG_HOME`. Changing it with
`home.sessionVariables` doesnt work, since it has to be exported
before Zplug is initialised
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.
Running `zplug install` will always product output, even if there is
nothing to do.
Gating it behind a `zplug check` eliminates that output when there is
nothing to do, and is recommended in the zplug README.
