Add message if conflicting contract on dependency install

[chasefleming]

There are cases where someone may have a conflicting contract name to a dependency or they may be migrating to use Dependency Manager. It would be nice to show a message if a conflict is found so they can address it (e.g. they may then want to remove it from their contracts section since it is now in Dependencies or rename their contract).

This is related to this (can be a stepping stone to it): #1544