FilesystemResourceDirectory does not handle zipped eggs

[do3cc]

files would probably be found when using pkg_resources api.

[do3cc]

I've looked a bit into it yesterday night nad figured out that the required changes are a bit of effort.
The FileSystemDirectoryLoader supports both loading from absolute paths and loading from packages.
This loader gets created by a zcml statement.
My preferred solution would be:

1. write a second FileSystemDirectoryLoader for files from eggs.
2. Let the zcml file dispatch to the proper Loader

While this looks backwards compatible, I found at least two instances where happens an isinstance check for the file loader. While I didn't look to close, I believe they then also use the os.path api to extract resources, so these packages would need to have code for handling this new FileSystemDirectoryLoader.