Fix bug in path handling of package listing / library location inference

Turns out there were a couple of bugs here -- one related to inconsistent file path separator use, and another due to the use of junction points.

1. We now ensure we consistently use the same path separator when determining 'in packrat library?' membership, and

2. We hack around the fact that `normalizePath` does not resolve junction points (neither does `Sys.readlink`) on Windows.

Re: https://github.com/rstudio/packrat/issues/133