adjust Makefile for go modules
adjust Makefile for go modules

hiroakis/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins