bugfix: Adding depth limit to resolveExprPath
bugfix: Adding depth limit to resolveExprPath

There is no termination condition for evaluation of cyclical
expression paths which can lead to infinite loops. This addresses
one spot in the parser in a similar fashion as utils.cc/canonPath
does.

This issue can be reproduced by something like:

```
ln -s a b
ln -s b a

nix-instantiate -E 'import ./a'
```

(cherry picked from commit d89d9958a771fb6299eae084c1b52d0ecf95901b)

dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix