Alternative option for allowing a user to automatically configure what
binary to use from a `pinentry` package. Previously, we always used
`meta.mainProgram` but, there are packages that provide multiple
binaries and this would allow flexibility for a user to override the
default program used.
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.
