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>
