compatibility fix with fpm path-mapping feature

FPM::Package::Dir expects paths to behave as strings.
The [path-mapping][] feature depends on regex comparisons with
path names.

Check the [error the PR addresses and a minimal recipe to reproduce][sample-gist]

[path-mapping]: https://github.com/jordansissel/fpm/commit/8360506c394b18e568d17b66e650639b46fbae7b
[sample-gist]: https://gist.github.com/skiold/f6b9da6664fc1c0067f4