We have two fields, `Repos` and `PackageSets`. Renaming
`PackageSetsRepositories` to `PackageSetsRepos` for consistency.
The struct is for internal use only so the rename has no impact as long
as the serialised name is the same (json tag).
Also it's shorter.
Added docstring to the struct that explains the arguments in the same
way as they are described for the `depsolve()` function.
Changing the name of the argument in the internal `depsolve()` function
for the same reasons.
82eedf5b82