Windows: Substituted paths are not resolved correctly

### General summary/comments
I'm trying to overcome the long paths issue (#3285) on Windows by using subst command for creating shorter paths from longer ones (similarly to mount on Linux).

### Steps to reproduce

For example:

1. subst p: c:\mylong...longpath\
2. p:
3. stack build
4. It doesn't build and fail with "copyFile: does not exist (No such file or directory)" because of the long paths.

### Expected

All paths (starting with "p:") are resolved to "c:\mylong...longpath\" instead to "p:". If they were resolved to "p:" it will not fail with long paths.

### Stack version

```
$ stack --version
Version 1.5.1, Git revision 600c1f01435a10d127938709556c1682ecfd694e x86_64 hpack-0.17.1
```

### Method of installation

* Official binary