Improve package structure

Rename `stirling.itch.bats`, `stirling.itch.nasdaq` and `stirling.itch.nasdaqomx` to `stirling.bats`, `stirling.nasdaq` and `stirling.nasdaqomx`, respectively.

While these packages currently contain exclusively non-FIX protocols, a similar relocation is due for FIX dialects in near future. Then the NASDAQ OMX FIX dialect, for example, will be found alongside other NASDAQ OMX protocols, such as ITCH and OUCH.