server: Create file paths as needed

server: Create file paths as needed

Add support for non-flat source file layouts (e.g. relative paths that
contain directory parts).

Instead of creating the cache directory and possible per-file
subdirectories separately, just create per-file directories on the
fly.  This simplifies the code, but means that you won't die until the
first request if your server doesn't have permission to create these
directories.