WIP: refactor(cluster): refactor cluster package

[justxuewei]

What this PR does:

The new directory structure is shown at below.

├── cluster              // abstract cluster
│   ├── cluster          // cluster interface & implementation
│   │   ├── available    // available cluster implementation
│   │   │   ├── ...
│   │   ├── base         // base cluster implementation
│   │   │   ├── ...
│   │   ├── broadcast    // broadcast cluster implementation
│   │   │   ├── ...
│   │   ├── failback     // failback cluster implementation
│   │   │   ├── ...
│   │   ├── failfast     // failfast cluster implementation
│   │   │   ├── ...
│   │   ├── failover     // failover cluster implementation
│   │   │   ├── ...
│   │   ├── failsafe     // failsafe cluster implementation
│   │   │   ├── ...
│   │   ├── forking      // forking cluster implementation
│   │   │   ├── ...
│   │   └── zoneaware    // zoneaware cluster implementation
│   │       ├── ...
│   ├── cluster_impl     // compatible with legacy imports, will be REMOVED in the future
│   │   └── import.go
│   ├── directory        // directory interface & implementation
│   │   ├── base
│   │   │   ├── ...
│   │   └── static
│   │       ├── ...
│   ├── loadbalance      // load balance interface & implementation
│   │   ├── consistenthashing   // consistenthashing load balance implementation
│   │   │   ├── ...
│   │   ├── leastactive  // leastactive load balance implementation
│   │   │   ├── ...
│   │   ├── random       // random load balance implementation
│   │   │   ├── ...
│   │   ├── roundrobin   // roundrobin load balance implementation
│   │   │   ├── ...
│   └── router           // NOT modified in this PR
│       ├── ...

PS: This refactor will be transparent for Dubbo-Go users.

Which issue(s) this PR fixes:

Fixes #1503

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Please note that importing "_ dubbo.apache.org/dubbo-go/v3/cluster/loadbalance" for loadbalance initialization couldn't work anymore.

Users must import used loadbalances respectively, or import `_ dubbo.apache.org/dubbo-go/v3/imports`(highly recommended way) instead.

Here are an example of importing all of loadbalances respectively:
import (
    _ "dubbo.apache.org/dubbo-go/v3/cluster/loadbalance/consistenthashing"
    _ "dubbo.apache.org/dubbo-go/v3/cluster/loadbalance/leastactive"
    _ "dubbo.apache.org/dubbo-go/v3/cluster/loadbalance/random"
    _ "dubbo.apache.org/dubbo-go/v3/cluster/loadbalance/roundrobin"
)