Add soldeer clean command

[ufoscout]

It would be useful to have a clean command to remove downloaded and generated artifacts, as is commonly available in other package managers.

[mario-eth]

we have uninstall once the new version is released, 0.3.0

[ufoscout]

@mario-eth
Maybe I'm wrong, but uninstall removes a dependency from the dependencies tree (i.e. it removes it from the soldeer.lock file and the dependencies folder). Instead clean would only delete the dependencies folder and any other temporary or generated artifact; then, if you use soldeer update or soldeer build those folders will be recreated from scratch.
Essentially soldeer clean should be the equivalent of cargo clean