Add convenience functions for determining canister / library entry points

[rvanasa]

import mo from 'motoko';

mo.write('Main.mo', 'actor {}');
mo.write('Other.mo', 'actor {}');

const mainPath = mo.resolveMain(); // 'Main.mo'

mo.delete('Main.mo');

const newPath = mo.resolveMain(); // 'Other.mo'