chore(sdk): A bit of code moving in event_cache

[Hywan]

This patch renames event_cache/store.rs to event_cache/room/events.rs, and extract types about RoomEventCache from event_cache/mod.rs to event_cache/room/mod.rs. I hope it clarifies things.

It's purely code moves, no renaming, no other changes.

[bnjbvr]

Thanks! There are some unit tests that would require some code motion too, please :-)

[codecov]

Codecov Report

Attention: Patch coverage is 91.16022% with 16 lines in your changes missing coverage. Please review.

Project coverage is 84.79%. Comparing base (77b3aa8) to head (b775df2).
Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/mod.rs	91.16%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4178      +/-   ##
==========================================
- Coverage   84.82%   84.79%   -0.04%     
==========================================
  Files         269      270       +1     
  Lines       28963    28961       -2     
==========================================
- Hits        24569    24558      -11     
- Misses       4394     4403       +9     

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[Hywan]

Indeed! Tests have been moved accordingly.