Dub has always been notoriously hard to test due to the amount of IO it does.
Over the past few years, I have progressively refactored it to allow
dependency injection to take place, which this finally put into action.
By overriding the `PackageManager`, `PackageSupplier`, and a few strategic
functions, we can start to unittest Dub's behavior solely in unittests.
This should hopefully tremendously helps with adding regression tests
for the package manager side of Dub (the build side still need work
to have similar capabilities).