[WIP] [Proposal] Rename stream submodules to improve readability

[peaBerberian]

While working on the Stream (the module and submodules determining which segments need to be loaded), I thought that the current architecture was relatively difficult to follow. For example, the submodules inside it are hierarchical and follow a StreamOrchestrator > PeriodStream > AdaptationStream > RepresentationStream path.

It makes relative sense when you know DASH and think a little about it, but I feel that it could be improved on.

For now, I'm not set on what we could name it, I just searched-and-replaced and tried different things.
Notably, directories inside src/core/stream are now numbered and indicates what job they have:

1-Period_selection/
2-Track_selection/   
3-Representation_selection/
4-Segment_selection/

(instead of orchestrator/, period/, adaptation/ and representation/ respectively)

And the modules themselves have also been renamed for what they do (e.g. TrackSelector) instead of the DASH abstraction level they're at like before (like PeriodStream). But I'm still not 100% happy with that.

This is background work not important for the other work I was doing (which is implementing more resilience on decoding errors) but it is something I saw so I thought I would open a PR so we can think about how we could improve on this.

[github-actions]

✅ Automated performance checks have passed on commit c45dc45894b19e9c1a20b7e716454a12e0807f9c with the base branch dev.

Details

Performance tests 1st run output

No significative change in performance for tests:

Name	Mean	Median
loading	25.74ms -> 25.92ms (-0.180ms, z: 0.06787)	36.45ms -> 36.45ms
seeking	319.92ms -> 319.27ms (0.653ms, z: 0.04397)	17.70ms -> 17.85ms
audio-track-reload	32.75ms -> 32.91ms (-0.165ms, z: 1.22615)	48.90ms -> 49.05ms
cold loading multithread	52.43ms -> 52.18ms (0.252ms, z: 8.47979)	77.25ms -> 76.20ms
seeking multithread	61.48ms -> 58.84ms (2.643ms, z: 0.14409)	20.70ms -> 20.70ms
audio-track-reload multithread	31.14ms -> 30.95ms (0.189ms, z: 2.51713)	45.90ms -> 45.60ms
hot loading multithread	20.78ms -> 20.70ms (0.082ms, z: 2.41988)	30.45ms -> 30.30ms