`Arbitrary` instances for `InstallDirs` and `PathTemplate` create invalid filenames

[geekosaur]

Describe the bug
While they are used at multiple types, ultimately those bottom out to Prelude's FilePath which is String. The result is that the pathnames they generate can contain NULs, Windows device names, and illegal sequences (for example, on Windows a path component cannot end with a .).

Ideally we would use the new filepath library with its bespoke FilePath type, for which valid Arbitrary instances can be written and may even already exist. There is no way to make it work with Prelude's FilePath.

There is more discussion in #9253 and #9254, and the latter adds a warning comment to the problematic instances.