remove GopEnv

Module.Lookup: add pkg.Real

fix cloneGoFileSyntax crash

Module.IsValid; LookupExternPkg

Update go.yml

modfetch.Split/GetPkg; rm modload.Mode; private gopmod.Module.lookupE…

…xternPkg

modfetch.Get fix: use @latest to download

mv GopEnv => env.Gop

lookup fix

Update README.md