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.
Note, the pubs configuration file uses ConfigObj syntax, which is
similar to the INI files syntax but with extra functionalities like
nested sections. This prevents it from using Nix's INI format
generator. Here is an example of pubs configuration that cannot be
generated using Nix's INI format generator:
[plugins]
[[git]]
manual=False
For this reason, we opted for a stringly-typed configuration since the
use of a structured `settings` option would require a custom parser.