chore(build): set build path to gopath/

With this, pkg/ could be used as normal directory, and it doesn't need
to gitignore pkg/, which shields pkg/http and pkg/strings.

.gitignore

@@ -1,7 +1,6 @@
-/pkg
+/gopath
 /go-bindata
 /machine*
 /bin
-/src
 .vagrant
 *.etcd

build

@@ -1,12 +1,12 @@
 #!/bin/sh -e
 
-if [ ! -h src/github.com/coreos/etcd ]; then
-	mkdir -p src/github.com/coreos/
-	ln -s ../../.. src/github.com/coreos/etcd
+if [ ! -h gopath/src/github.com/coreos/etcd ]; then
+	mkdir -p gopath/src/github.com/coreos/
+	ln -s ../../../.. gopath/src/github.com/coreos/etcd
 fi
 
 export GOBIN=${PWD}/bin
-export GOPATH=${PWD}
+export GOPATH=${PWD}/gopath
 export GOFMTPATH="./bench ./config ./discovery ./etcd ./error ./http ./log main.go ./metrics ./mod ./server ./store ./tests"
 
 # Don't surprise user by formatting their codes by stealth