revert postgres package refactor (#1796)

* Revert "refactor: move postgres to pg package (#1781)"

This reverts commit ba8ac974aa.

* remove pg, use postgres

core/stores/postgres/postgresql.go

@@ -1,7 +1,14 @@
 package postgres
 
-import "github.com/zeromicro/go-zero/core/stores/pg"
+import (
+	// imports the driver, don't remove this comment, golint requires.
+	_ "github.com/lib/pq"
+	"github.com/zeromicro/go-zero/core/stores/sqlx"
+)
 
-// New creates a new postgresql store.
-// Deprecated: use pg.New instead.
-var New = pg.New
+const postgresDriverName = "postgres"
+
+// New returns a postgres connection.
+func New(datasource string, opts ...sqlx.SqlOption) sqlx.SqlConn {
+	return sqlx.NewSqlConn(postgresDriverName, datasource, opts...)
+}