StandardJavaFileManager should provide a way to get paths from strings

Although most uses of Path can be specified through the StandardJavaFileManager API, there are some remaining uses, most notably in options, where a string has to be converted to a Path.

Currently, this is done with Paths.get, but that presumes the use of the default file system, which may not be appropriate in all environments.

StandardJavaFileManager should provide a way for the client to specify how to map a string to a Path.