[Refactor] Move rockspec/src.rock downloading logic to RemotePackage impl

[mrcjkb]

...since we may want to support other servers than luarocks.
None of it should be exposed by rocks-lib.

We might want to move RemotePackage to its own module.

[vhyrro]

With #285, maybe we should let RemotePackageDB implement various aspects like this? I'm thinking of some trait that one could implement for downloading, querying etc. Then the RemotePackageDB would operate on this trait.

[mrcjkb]

Sounds good to me. I think it's just a question of moving the logic in the operations::download module to the RemotePackageDB impl.
We might not even have to expose RemotePackage.

[vhyrro]

That's a good point, library consumers will only really need LocalPackages and PackageReqs for daily use.