plugins: when loading plugins, use newer importlib mechanism
plugins: when loading plugins, use newer importlib mechanism

fixes #4842

neocogent/electrum,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,fujicoin/electrum-fjc,spesmilo/electrum,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,vialectrum/vialectrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,pooler/electrum-ltc,fujicoin/electrum-fjc,neocogent/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,neocogent/electrum,fujicoin/electrum-fjc,vialectrum/vialectrum,vialectrum/vialectrum