Refactor: 1/n legacy accelerators and plugins

[Borda]

What does this PR do?

moving actual accelerators and plugins to its legacy sub-package to not interfere with the new structure
This is the initial step for #5616

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #5645 (bbbe7c6) into release/1.2-dev (9d165f6) will decrease coverage by 3%.
The diff coverage is 100%.

@@                Coverage Diff                @@
##           release/1.2-dev   #5645     +/-   ##
=================================================
- Coverage               93%     89%     -3%     
=================================================
  Files                  154     163      +9     
  Lines                11040   12070   +1030     
=================================================
+ Hits                 10216   10795    +579     
- Misses                 824    1275    +451