Previously, maildir path was not escaped in any way, so using a path with commas
lead to a TypeError:
```
TypeError: expected string or bytes-like object, got 'list'
```
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.
