Always use original location instead of shadow-copied location when loading RProvider plugins

This may or may not help fix #122 . I've used this snippet many times before (e.g., in the facio test runner) to solve a similar issue. I wasn't able to build/run the RProvider tests on my machine -- the FAKE build failed with some odd errors -- so someone will need to check whether this PR actually fixes the issue as expected.
AFAIR `System.Uri` won't handle special characters (i.e. `#`) in paths:
```
> open System;;
> let path = "file:///d:/F#/dir1/dir2";;

val path : string = "file:///d:/F#/dir1/dir2"

> let localPath = Uri(path).LocalPath;;

val localPath : string = "d:\F"
```