Integrate new search path algorithm

[ysbaddaden]

We must follow the search patch changes of crystal-lang/crystal#2788. This is a breaking change; projects will have to run rm -rf libs; shards install in their projects for the new dependencies to be installed correctly.

• install dependencies in lib instead of libs;
• install dependencies as a whole (not merely the src folder).

[ysbaddaden]

This is blocking #95 (and possibly #110) from being implemented, since the bin folder of dependencies isn't available.

[RX14]

Should shards detect the old libs format and delete it before install? Have a marker file? Is that too much work for little gain?

[ysbaddaden]

Maybe print a message? Maybe have shards prune remove it? Or ask for permission? I'm never comfortable deleting files on someone's computer...

[RX14]

Maybe print a message saying it's in the old format and must be removed, then exit.