Remove `logfire._internal` from `logfire-api`

[samuelcolvin]

Having the _internal sub-package is great as it provides a boundary to not include in the logfire-api package. We should remove it, it would also make the logfire-api package lighter.

[Kludex]

I don't think we can do that unless we move the types around in logfire itself. We use mypy stubgen, and we purposely include internal to not lose some types.

Currently the logfire-api wheel size is 73.9 kB.