Install Addins, Tools, Modules to global packages folder

To avoid unnecessary installation of packages already located in global packages folder (e.g. `~/.nuget`), we should install and consume packages from there. We could make it opt-in to start with.

One problem I see is tool resolution. We'd probably need a way to register search paths for installed tools. Then tool installer could register the path for the installed tool, located in global packages folder.

Module resolution might be tricky one... 😄

Start by tracing the tool installer and the existing package and module resolution entry points; the issue does not name specific files or tests. Determine how an opt-in global packages folder would be consumed, how installed tool paths could be registered, and how module resolution would behave. Done means the proposal's package, tool, and module resolution behavior is defined and covered by appropriate tests.

Written by the indexing model from the issue text.