The current `//` override to `agda.passthru.tests` is non-recursive so
it destroys everything else under `passthru`, and furthermore does not
go through `mkDerivation` so that we end up with different values for
`agda.tests` and `agda.passthru.tests`.
Fix it by moving the `allPackages` test to the definition of
`withPackages`.
The Everthing module is not part of a library and should therefore
not be copied to the nix store.
This is particularly bad, if the Everything module is defined in
an agda library included directory, e.g. consider an agda-lib with
include: .
and Everything.agda in the project root (.), in which case the
Everything module would become part of the library.
If multiple such projects are in the dependency tree, the Everything
module becomes ambiguous and the build would fail.
