net/nats-streaming-server: Unbreak with go1.13

===>  Building for nats-streaming-server-0.15.0
go: github.com/go-sql-driver/mysql@v1.4.1: Get https://proxy.golang.org/github.com/go-sql-driver/mysql/@v/v1.4.1.mod: dial tcp: lookup proxy.golang.org on 209.222.18.218:53: write udp 127.0.0.1:51039->209.222.18.218:53: write: permission denied
*** Error code 1

- Switch to USES=go:modules
- Remove GO_PKGNAME, not needed with modules

PR:		240314
Submitted by:	Dmitri Goutnik <dg@syrec.org>
Approved by:	olgeni (maintainer)
Pointy hat:	jlaffaye

net/nats-streaming-server/Makefile

@@ -11,10 +11,7 @@ COMMENT=	Streaming system server for NATS
 LICENSE=	MIT
 LICENSE_FILE=	${WRKSRC}/LICENSE
 
-BROKEN=		fails to build
-
-USES=		go
-GO_PKGNAME=	github.com/${GH_ACCOUNT}/${PORTNAME}
+USES=		go:modules
 USE_GITHUB=	yes
 GH_ACCOUNT=	nats-io