Use the name "index.json" instead of "bl_ext_repo.json".
This makes more sense as the name is automatically added to the end
of file:// URL's a little like index.html for web-sites.
Manually deleting the extensions directory caused checking for
updates on startup to fail.
It attempted to create a directory without the parent existing
which raised an error.
Previously regular paths were supported however supporting both
URL's and file paths isn't very common & complicated internal logic.
Also fixed an error where the server-generate sub-command created the
"archive_url" from the file-name without URL encoding.
