Enhancement of external plugin enabling (#1392)

This fix is an enhancement of external plugin enabling.
Previously, it was already able to build a customerized
coredns with plugins enabled selectively, without changing
coredns source code. However, all default plugins are
actually bundled because of the import rule:
```
"github.com/coredns/coredns/coremain"
```

The issue is best described with the following:
```
root@localhost:/go/src/github.com/coredns/coredns/sample# cat sample.go
package main

import (
        _ "github.com/coredns/forward"

        "github.com/coredns/coredns/coremain"
        "github.com/coredns/coredns/core/dnsserver"
)

var directives = []string{
        "forward",
        "startup",
        "shutdown",
}

func init() {
        dnsserver.Directives = directives
}

func main() {
        coremain.Run()
}
root@localhost:/go/src/github.com/coredns/coredns/sample#
root@localhost:/go/src/github.com/coredns/coredns/sample# go build -v sample.go
root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins
root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins
Server types:
  dns

Caddyfile loaders:
  flag
  default

Other plugins:
  dns.auto
  dns.autopath
  dns.bind
  dns.cache
  dns.chaos
  dns.debug
  dns.dnssec
  dns.dnstap
  dns.erratic
  dns.errors
  dns.etcd
  dns.federation
  dns.file
  dns.forward
  dns.health
  dns.hosts
  dns.kubernetes
  dns.loadbalance
  dns.log
  dns.nsid
  dns.pprof
  dns.prometheus
  dns.proxy
  dns.reverse
  dns.rewrite
  dns.root
  dns.route53
  dns.secondary
  dns.template
....
```

This fix moves zplugins.go to a different package/directory so that
it is possible to "only import plugins as needed".

The following is the new output after this fix:
```
root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins
Server types:
  dns

Caddyfile loaders:
  flag
  default

Other plugins:
  dns.forward
  dns.prometheus
  shutdown
  startup

root@localhost:/go/src/github.com/coredns/coredns/sample#
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

core/plugin/zplugin.go

@@ -1,6 +1,6 @@
 // generated by directives_generate.go; DO NOT EDIT
 
-package core
+package plugin
 
 import (
 	// Include all plugins.

coredns.go

@@ -2,7 +2,12 @@ package main
 
 //go:generate go run directives_generate.go
 
-import "github.com/coredns/coredns/coremain"
+import (
+	"github.com/coredns/coredns/coremain"
+
+	// Plug in CoreDNS
+	_ "github.com/coredns/coredns/core/plugin"
+)
 
 func main() {
 	coremain.Run()

coremain/run.go

@@ -15,9 +15,6 @@ import (
 	"github.com/mholt/caddy"
 
 	"github.com/coredns/coredns/core/dnsserver"
-
-	// Plug in CoreDNS
-	_ "github.com/coredns/coredns/core"
 )
 
 func init() {

directives_generate.go

@@ -48,7 +48,7 @@ func main() {
 		}
 	}
 
-	genImports("core/zplugin.go", "core", mi)
+	genImports("core/plugin/zplugin.go", "plugin", mi)
 	genDirectives("core/dnsserver/zdirectives.go", "dnsserver", md)
 }
 

test/plugin_test.go

@@ -8,6 +8,9 @@ import (
 	"github.com/coredns/coredns/plugin/test"
 
 	"github.com/miekg/dns"
+
+	// Load all managed plugins in github.com/coredns/coredns
+	_ "github.com/coredns/coredns/core/plugin"
 )
 
 func benchmarkLookupBalanceRewriteCache(b *testing.B) {