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.
When setting values using the `git config --set` command, git formats
the file a bit differently. This changes the output so it maps to that
format.
Differences:
* each `key = value` in a section is prefixed by a tab character
* the `=` between the key and the value is surrounded by spaces
PR #1069
