#5498

This moves the various stream implementations into dedicated sub-packages, for having a cleaner separation of different stream types, as well as for being able to define the BaseSegment in the upcoming PR in a dedicated module. This PR therefore basically just moves the module files and updates the import paths (and mock paths in tests). I decided against naming the individual modules of those sub-packages with a leading underscore character.

As a side effect of moving the modules, and I'm not sure whether I like it, the logger names have changed as well (due to the __name__ reference). Example: stream.hls -> stream.hls.hls and stream.hls_playlist -> stream.hls.m3u8.

I believe it'd be better to override/revert the redundant logger names, which are stream.segmented.segmented, stream.hls.hls and stream.dash.dash. The logger names of the parser modules (hls.m3u8 and dash.manifest) are fine IMO and should be kept. They will be different no matter what due to the modules being renamed (previously hls_parser and dash_manifest).

In regards to the exports of the sub-packages, there will be no difference for the segmented stream base classes. The HLS sub-package also exports most of the m3u8 module, which I don't think is necessary, but I added those exports anyway since there are lots of custom HLS implementations. The DASH sub-package however only exports the relevant stuff. If there's a need for plugins to subclass DASH internals for custom implementations, then any missing stuff can be added later.