Refactor server subcommand directory structure

[kangasta]

This syncs directory structure with command structure.

Developed on top of #142, thus the non-main target. Different branch/PR to keep the Files changed tab cleaner.

Tree would change into:

$ tree internal/commands/server/
internal/commands/server/
├── create.go
├── create_test.go
├── delete.go
├── delete_test.go
├── eject.go
├── eject_test.go
├── firewall
│   ├── create.go
│   ├── create_test.go
│   ├── delete.go
│   ├── delete_test.go
│   ├── server_firewall.go
│   ├── show.go
│   └── show_test.go
├── list.go
├── list_test.go
├── load.go
├── load_test.go
├── modify.go
├── modify_test.go
├── networkinterface
│   ├── create.go
│   ├── create_test.go
│   ├── delete.go
│   ├── delete_test.go
│   ├── modify.go
│   ├── modify_test.go
│   └── network_interface.go
├── plan_list.go
├── restart.go
├── restart_test.go
├── server.go
├── show.go
├── show_test.go
├── start.go
├── start_test.go
├── stop.go
├── stop_test.go
└── storage
    ├── attach.go
    ├── attach_test.go
    ├── detach.go
    ├── detach_test.go
    ├── server_storage.go
    └── server_storage_test.go

3 directories, 42 files