Added realpath for $this->repoDir assignment

If you have a repository entry with a url like so: "file://../localLibrary/" and run 'composer update' (or install) the following happens:

```
Failed to execute git clone "file://../localLibrary" "E:\CODE\project\vendor/localLibrary" 
&& cd "E:\CODE\project\vendor/localLibrary" 
&& git remote add composer "file://../localLibrary" 
&& git fetch composer

fatal: 'C:/Program Files (x86)/Git/mpl' does not appear to be a git repository
fatal: Could not read from remote repository.
```

The path becomes relative to git itself when the remote is added, thus we should use the full path resolved by realpath()