cannot create subnets separately from network itself

[ideasculptor]

Because the team(s) that have permission to modify networks in my architecture may differ from the team(s) that can perform other work, I really prefer to be able to declare subnets separately from declaring the network itself and making the project a VPC host project.

Declaring the network and host project alone is easy enough - just set var.subnets to []

However, it would be nice to be able to use this module to declare the subnets and routes, rather than copying those 2 resources into a module of their own. I don't see any obvious reason not to make those first 2 resources in main.tf optional, based on variables passed in, but before I put in the work of making a PR, it'd be nice to know if that change will be accepted - I don't know if that's an operating model google is interested in supporting.

The subnet and route resources really only use the network name, so it's very low-impact change to make, since that is already a variable. We just need to indicate whether we should create a network and then refer to that resource when creating subnets and routes or just use the passed-in network name (in my case, the passed-in name would be populated by remote_state that retrieves the network_name that got assigned when the network itself was created).

[morgante]

I don't think this is a common scenario, but since its a low-impact change I don't mind adding it.