fix(duplicate files): always result in unique file names

[nicojs]

Make sure that the typescript transpiler plugin always results in unique
file names. For example, given the following input:

foo.js
foo.ts

It will transpile foo.ts to foo.js, the result is now:

foo.js

Fixes #1318

[bartekleon]

@nicojs instead of using object you could try Set (sth like array) or Map (sth like object), they don't allow same elements to be added :/

[nicojs]

@kmdrGroch a Set would not be useful. Objects are by their nature always unique. A Map could work. The problem is that they don't serialize easily (although we're using surrial which can serialize anything).

What are the advantages of using a map over an object?

[bartekleon]

@nicojs Well Map has better / cleaner syntax than an object, there are also some articles about it if you are interested:
https://medium.com/front-end-weekly/es6-map-vs-object-what-and-when-b80621932373
But it's really up to you which one you prefer