stack-to-nix cannot correctly parse packages with revision

for example
```
hsimport-0.11.0
```
works fine whereas
```
hsimport-0.11.0@rev1
```
will produce this error
```
stack-to-nix: ./hsimport-0.11.0@rev1: getDirectoryContents:openDirStream: does not exist (No such file or directory)
```
