Apisix-dashboard: [style] adjust directory structure of api to golang standard style

[gxthrj]

ref to #715

[membphis]

@gxthrj please set right lable

[membphis]

please check the other issues you have created: label and milestone

[juzhiyuan]

2.1 is ok for me, but not sure if it's a big project to reactor api's directory at that time 🤔

[tokers]

A popular go project layout is like:

.
├── build-tools
├── cmd
│   └── manager-api
├── conf # default config path
├── logs
├── output
│   ├── conf
│   └── dag-to-lua
├── pkg
│   ├── config
│   ├── entity
│   ├── filter
│   ├── handler
│   │   ├── authentication
│   │   ├── consumer
│   │   ├── healthz
│   │   ├── plugin
│   │   ├── route
│   │   ├── service
│   │   ├── ssl
│   │   └── upstream
│   ├── log
│   ├── storage
│   ├── store
│   └── utils
│       └── consts
└── test
    ├── docker
    ├── e2e
    └── shell

Note this is only a demonstration not the ultimate one.

[juzhiyuan]

related resources: https://github.com/golang-standards/project-layout

[tokers]

Also, we can obey the uber-go style guide.