fix #28

[soraxas]

This PR adds:

• perform unlink/relink when upgrading a package
• It also performs upgrade on its dependencies.
• Exit upgrade gracefully if it's not upgradable (using same logic as outdated) so that unlinking/relinking won't be performed.

This addresses #28

[juanibiapina]

Again, thanks for your contributions 😄 .

There's two set of changes here, and also it's missing tests. Would you still care to split it and add tests?