Be more specific when looking for physics plugins

[chapulina]

Summary

This came up on #617 while experimenting with different ways of loading plugins. More specifically, we were trying to use the RTLD_GLOBAL flag to load plugins, which made the plugin loader find all plugins already loaded, not only those from the current library. In that situation, it was useful to filter plugins by interface before iterating over them.

We ended up not using that flag, but I think it's a good idea to filter the plugins beforehand anyway in case a user tries to load some funky plugin.

Test it

I'd expect this change not to break any of the existing tests.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #965 (1ea86b5) into ign-gazebo3 (b7346a4) will decrease coverage by 0.01%.
The diff coverage is 20.00%.

@@               Coverage Diff               @@
##           ign-gazebo3     #965      +/-   ##
===============================================
- Coverage        77.90%   77.88%   -0.02%     
===============================================
  Files              221      221              
  Lines            12678    12681       +3     
===============================================
  Hits              9877     9877              
- Misses            2801     2804       +3     

Impacted Files	Coverage Δ
src/systems/physics/Physics.cc	70.34% <20.00%> (-0.25%)	⬇️

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