Move pulumi new "install dependencies" implementation into the language plugins

[justinvp]

This is implemented inside pulumi new for a simpler initial implementation, but it really should live inside the language plugins long-term. Then pulumi new would just call the language plugin (e.g. InstallDependencies or PrepareProject) to install dependencies, and the language plugin would handle doing whatever it needs to do. That way, if/when we add new languages, pulumi new would just work with the new language.

Since there is a gRPC boundary between the CLI and language plugin, the tricky part will be making sure we can show the Stdout and Stderr of any commands (e.g. npm install) run by the language plugin.

[justinvp]

We have a similar issue for Policy Packs. We need to be able to package a policy pack and then subsequently install it and its dependencies. Right now we effectively have switch statements, but really we should be deferring to the analyzer plugins to do this.