Fix trailing slash handling in pkg_resources.ZipProvider

Given a ZipProvider, if the underlying ZipImporter prefix is empty,
then resource_listdir('') and resource_listdir('subdir/') fail, while
resource_listdir('/') and resource_listdir('subdir') succeed.

On the other hand, if the underlying ZipImport prefix is not empty,
then resource_listdir('/') fails but resource_listdir('') succeeds.

With this change, the cases listed succeed with or without trailing
slashes.

pkg_resources/__init__.py

@@ -1693,6 +1693,10 @@ class ZipProvider(EggProvider):
     def _zipinfo_name(self, fspath):
         # Convert a virtual filename (full path to file) into a zipfile subpath
         # usable with the zipimport directory cache for our target archive
+        while fspath.endswith(os.sep):
+            fspath = fspath[:-1]
+        if fspath == self.loader.archive:
+            return ''
         if fspath.startswith(self.zip_pre):
             return fspath[len(self.zip_pre):]
         raise AssertionError(