fix #11625: sanitize invalidly named FS paths

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/11625 by partially reverting https://github.com/mtbc/openmicroscopy/commit/34748dda2620ce1e5d4b2c5111a9804d85367bd2. Now Python clients should be able to set `FilesetEntry.clientPath` to a `/`-separated but otherwise-hairy path (e.g., `"/".join(dir.splitall())`) and have the server fix the path at import time so as to make it sanitary. (`ImportLibrary.createImport` has `ImportContainer.fillData` do path sanitization but clients may import by other means.) The `FilePathNamingValidator` class, though now unused, may yet have further use and is very lovely, so remains in the codebase.
--no-rebase
I wondered if there was some nice Guava way to transform the elements of a collection (`map (map f) ...`) but I don't easily see one.