Use the current workdir as reference to calculate the expanded_path of a filename

This resolves issues with autoloader layout detection when the module dir
is a symlink.

```
.
├── foobar -> foo-bar/
└── foo-bar
    └── manifests
            ├── foo.pp
            └── init.pp
```

Running puppet-lint in ./foobar results in:

```
WARNING: class not documented on line 1
ERROR: foobar not in autoload module layout on line 1
WARNING: class not documented on line 1
ERROR: foobar::foo not in autoload module layout on line 1
```