Running `spago init` twice overwrites `packages.dhall`

[menelaos]

Steps to reproduce:

1. spago init
2. Modify packages.dhall
3. spago init

The second spago init overwrites the modified packages.dhall file.
It looks like the bug was introduced in this PR (changing die to echo in makePackageSetFile).

[f-f]

Fix up in #321