fix: maybeRelativePath: use posix separator

[lanycrost]

Hey there!

Because of the changes made here. Imports working incorrect.

It will affect https://github.com/stephenh/ts-proto. So please fix it. It's already affecting us.

Here is explanation.

import "google/protobuf/timestamp.proto";
...

on Windows resulting path:

import { Timestamp } from '..googleprotobuf\timestamp'; 

should be:

import { Timestamp } from '../google/protobuf/timestamp';

Here is the fix of issue mentioned here

https://github.com/stephenh/ts-poet/issues/32

[stephenh]

Thanks @lanycrost !