Normalize relative external paths. - #591

I've encountered a problem in such a scenario:

• I had a path to a module declared as an external (so that the file would be imported later, not by rollup)
• a file that was not in the same directory as the entry file was importing that module (../../module)

It turned out that the imported module in the bundle was still ../../module, while I expected it to be normalized with respect to, let's say, the entry file.

I investigated a bit and found out something scary: if a relative module is imported from many files in a nested structure, there will be more than one import in the resulting bundle.

For example, I get this:

var foo = _interopDefault(require('./foo'));
var foo$1 = _interopDefault(require('../foo'));
var foo$2 = _interopDefault(require('../../foo'));

var first = foo$1 + foo$2;
var main = foo + first;
module.exports = main;

instead of this:

var foo = _interopDefault(require('./foo.js'));

var first = foo + foo;
var main = foo + first;
module.exports = main;

This PR fixes that problem by normalizing the paths with respect to the resolved entry directory.

I probably did some terrible mistakes because I don't fully understand what's happening there. So please be gentle 😉