loader: load packages using Go modules

This commit replaces the existing ad-hoc package loader with a package
loader that uses the x/tools/go/packages package to find all
to-be-loaded packages.

main.go

@@ -733,7 +733,7 @@ func printCompilerError(logln func(...interface{}), err error) {
 			}
 		}
 	case loader.Errors:
-		logln("#", err.Pkg.ImportPath)
+		logln("#", err.Pkg.PkgPath)
 		for _, err := range err.Errs {
 			printCompilerError(logln, err)
 		}