caddyplug reports plugins installed, but caddy does not

Info:

(with go111module=on)

```
yak@ubuntu-yak:~$ go version
go version go1.12.7 linux/amd64
```

```
yak@ubuntu-yak:~$ caddyplug list
fetching missing dependencies: caddy, dnsproviders, hook.pluginloader
go: finding github.com/abiosoft/caddyplug latest
done fetching depedencies.

http:
✓ grpc
...
```

```
yak@ubuntu-yak:~$ caddyplug install-caddy
fetching missing dependencies: caddy, dnsproviders, hook.pluginloader
go: finding github.com/abiosoft/caddyplug latest
done fetching depedencies.

installing Caddy...
go: finding github.com/caddyserver/caddy/caddy latest
✓ installed Caddy in /home/yak/go/bin/caddy
yak@ubuntu-yak:~$ /home/yak/go/bin/caddy -plugins
Server types:
http

Caddyfile loaders:
short
flag
default

Other plugins:
http.basicauth
http.bind
http.browse
http.errors
http.expvar
http.ext
http.fastcgi
http.gzip
http.header
http.index
http.internal
http.limits
http.log
http.markdown
http.mime
http.pprof
http.proxy
http.push
http.redir
http.request_id
http.rewrite
http.root
http.status
http.templates
http.timeouts
http.websocket
on
tls
tls.cluster.file
```