Fix FindSharedLibrary for relative plugin paths

[ahcorde]

Signed-off-by: ahcorde [email protected]

🦟 Bug fix

Summary

If you are running gazebo in the same folder that a custom plugin is located, then you are able to find it. According with the comment I think this code is more appropriate.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #382 (4570d80) into ign-common3 (eda17f5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           ign-common3     #382   +/-   ##
============================================
  Coverage        76.63%   76.63%           
============================================
  Files               73       73           
  Lines            10424    10425    +1     
============================================
+ Hits              7988     7989    +1     
  Misses            2436     2436           

Impacted Files	Coverage Δ
src/SystemPaths.cc	88.08% <100.00%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eda17f5...4570d80. Read the comment docs.