refactor: move postgres to pg package (#1781)

core/stores/pg/postgresql.go

@@ -0,0 +1,14 @@
+package pg
+
+import (
+	// imports the driver, don't remove this comment, golint requires.
+	_ "github.com/lib/pq"
+	"github.com/zeromicro/go-zero/core/stores/sqlx"
+)
+
+const postgresDriverName = "postgres"
+
+// New returns a postgres connection.
+func New(datasource string, opts ...sqlx.SqlOption) sqlx.SqlConn {
+	return sqlx.NewSqlConn(postgresDriverName, datasource, opts...)
+}