Implement plugins for bootstrap go modules

Now that we have multi-stage bootstrapping, we can make the primary
builder build more dynamic. Add the concept of plugins that will be
linked and loaded into bootstrap_go_binary or bootstrap_go_package
modules. It's expected that the plugin's init() functions will do
whatever registration is necessary.

Example Blueprint definition:

    bootstrap_go_binary {
      name: "builder",
      ...
    }

    bootstrap_go_package {
      name: "plugin1",
      pluginFor: ["builder"],
    }

A package may specify more than one plugin if it will be inserted into
more than one go module.

Change-Id: I109835f444196b66fc4018c3fa36ba0875823184