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.
Borgmatic has support for Borg's pattern matching. It is mutually
exclusive with the existing `sourceDirectories` option, so assertions
have been added to make sure that both are not set at the same
time (but also that at least one of them is). Additionally, tests have
been added to test the following configurations: `patterns` instead of
`sourceDirectories`, both at the same time, and neither.