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

There was logic in this block of code that relied on e.g.
`file.path("foo", "")` return a path with a trailing slash, e.g. "foo/";
however, on Windows a trailing slash is never returned by `file.path`
(documented in `?file.path`). To preserve downstream logic on Windows,
we manually append the path separator, and also shim normalizePath to
use `/` rather than `\` as the path separator.

Re: https://github.com/rstudio/packrat/issues/133