refactor: split utilities in separate files (#624)

Co-authored-by: Ward Peeters <ward@coding-tech.com>

Author: developit

src/cli.js

@@ -1,6 +1,6 @@
 #!/usr/bin/env node
 
-import microbundle from '.';
+import microbundle from './index';
 import prog from './prog';
 import { stdout } from './utils';
 import logError from './log-error';