Extract generate_simplestreams from tab2streams
Extract generate_simplestreams from tab2streams, so that it can be reused.
Separating out FileNamer may seem strange, but Juju uses a distinct naming
scheme for streams:
1. The proper index file is called index2.json (index.json is a cut-down
version with only one content-id due to limitations of old jujus)
2. The content files are not named exactly after the content-ids.
Instead, ':' is replaced with '-', because ':' is a special character
on Windows.